test(python): use constraints files to check dependency lower bounds (#869)

Use a constraints file when installing dependencies for system and unit tests nox sessions.

https://pip.pypa.io/en/stable/user_guide/#constraints-files
> Constraints files are requirements files that **only control which version of a requirement is installed, not whether it is installed or not**. Their syntax and contents is nearly identical to Requirements Files. There is one key difference: Including a package in a constraints file does not trigger installation of the package.

```
testing
├── constraints-3.10.txt
├── constraints-3.11.txt
├── constraints-3.6.txt
├── constraints-3.7.txt
├── constraints-3.8.txt
└── constraints-3.9.txt
```
  
Going forward, one constraints file (currently 3.6) will be populated with every library requirement and extra listed in the `setup.py`. The constraints file will pin each requirement to the lower bound. This ensures that library maintainers will see test failures if they forget to update a lower bound on a dependency.

See googleapis/python-bigquery#263 for an example

Author: busunkim96

synthtool/gcp/templates/python_library/noxfile.py.j2

@@ -18,6 +18,7 @@
 
 from __future__ import absolute_import
 import os
+import pathlib
 import shutil
 
 import nox
@@ -30,6 +31,8 @@ DEFAULT_PYTHON_VERSION="{{ default_python_version }}"
 SYSTEM_TEST_PYTHON_VERSIONS=[{% for v in system_test_python_versions %}"{{v}}"{% if not loop.last %},{% endif %}{% endfor %}]
 UNIT_TEST_PYTHON_VERSIONS=[{% for v in unit_test_python_versions %}"{{v}}"{% if not loop.last %},{% endif %}{% endfor %}]
 
+CURRENT_DIRECTORY = pathlib.Path(__file__).parent.absolute()
+
 # 'docfx' is excluded since it only needs to run in 'docs-presubmit'
 nox.options.sessions = [
     "unit",
@@ -88,24 +91,27 @@ def lint_setup_py(session):
 def default(session):
     # Install all test dependencies, then install this package in-place.
 
+    constraints_path = str(
+        CURRENT_DIRECTORY / "testing" / f"constraints-{session.python}.txt"
+    )
     {%- if microgenerator %}
-    session.install("asyncmock", "pytest-asyncio")
+    session.install("asyncmock", "pytest-asyncio", "-c", constraints_path)
     {% endif %}
-    session.install("mock", "pytest", "pytest-cov", {% for d in unit_test_external_dependencies %}"{{d}}"{% if not loop.last %},{% endif %}{% endfor %})
-    {% for dependency in unit_test_local_dependencies %}session.install("-e", "{{dependency}}")
+    session.install("mock", "pytest", "pytest-cov", {% for d in unit_test_external_dependencies %}"{{d}}",{% endfor %} "-c", constraints_path)
+    {% for dependency in unit_test_local_dependencies %}session.install("-e", "{{dependency}}", "-c", constraints_path)
     {% endfor %}
-    {% for dependency in unit_test_dependencies %}session.install("-e", "{{dependency}}"){% endfor %}
+    {% for dependency in unit_test_dependencies %}session.install("-e", "{{dependency}}", "-c", constraints_path){% endfor %}
     {%- if unit_test_extras_by_python %}
     {% for extras_python in unit_test_extras_by_python %}
     {%- if not loop.first %}el{% endif %}if session.python == "{{extras_python}}":
         extras = "[{{",".join(unit_test_extras_by_python[extras_python])}}]"
     {% endfor %}else:
         extras = "{%- if unit_test_extras %}[{{",".join(unit_test_extras)}}]{% endif %}"
-    session.install("-e", f".{extras}")
+    session.install("-e", f".{extras}", "-c", constraints_path)
     {% elif unit_test_extras %}
-    session.install("-e", ".[{{",".join(unit_test_extras)}}]")
+    session.install("-e", ".[{{",".join(unit_test_extras)}}]", "-c", constraints_path)
     {% else %}
-    session.install("-e", ".")
+    session.install("-e", ".", "-c", constraints_path)
     {% endif %}
 
     # Run py.test against the unit tests.
@@ -132,6 +138,9 @@ def unit(session):
 @nox.session(python=SYSTEM_TEST_PYTHON_VERSIONS)
 def system(session):
     """Run the system test suite."""
+    constraints_path = str(
+        CURRENT_DIRECTORY / "testing" / f"constraints-{session.python}.txt"
+    )
     system_test_path = os.path.join("tests", "system.py")
     system_test_folder_path = os.path.join("tests", "system")
 
@@ -156,10 +165,10 @@ def system(session):
 
     # Install all test dependencies, then install this package into the
     # virtualenv's dist-packages.
-    session.install("mock", "pytest", "google-cloud-testutils", {% for d in system_test_external_dependencies %}"{{d}}"{% if not loop.last %},{% endif %}{% endfor %})
+    session.install("mock", "pytest", "google-cloud-testutils"{% for d in system_test_external_dependencies %}, "{{d}}"{% endfor %}, "-c", constraints_path)
 
     {%- if system_test_local_dependencies %}
-    {% for dependency in system_test_local_dependencies %}session.install("-e", "{{dependency}}")
+    {% for dependency in system_test_local_dependencies %}session.install("-e", "{{dependency}}", "-c", constraints_path)
     {% endfor %}
     {%- endif %}
     {%- if system_test_extras_by_python %}
@@ -168,11 +177,11 @@ def system(session):
         extras = "[{{",".join(system_test_extras_by_python[extras_python])}}]"
     {% endfor %}else:
         extras = "{%- if system_test_extras %}[{{",".join(system_test_extras)}}]{% endif %}"
-    session.install("-e", f".{extras}")
+    session.install("-e", f".{extras}", "-c", constraints_path)
     {% elif system_test_extras %}
-    session.install("-e", ".[{{",".join(system_test_extras)}}]")
+    session.install("-e", ".[{{",".join(system_test_extras)}}]", "-c", constraints_path)
     {% else %}
-    session.install("-e", ".")
+    session.install("-e", ".", "-c", constraints_path)
     {% endif %}
 
     # Run py.test against the system tests.

tests/test_python_library.py

@@ -27,43 +27,43 @@
 @pytest.mark.parametrize(
     ["template_kwargs", "expected_text"],
     [
-        ({}, ["import nox", 'session.install("-e", ".")']),
+        ({}, ["import nox", 'session.install("-e", ".", "-c", constraints_path)']),
         (
             {"unit_test_local_dependencies": ["../testutils", "../unitutils"]},
             [
-                'session.install("-e", "../testutils")',
-                'session.install("-e", "../unitutils")',
+                'session.install("-e", "../testutils", "-c", constraints_path)',
+                'session.install("-e", "../unitutils", "-c", constraints_path)',
             ],
         ),
         (
             {"system_test_local_dependencies": ["../testutils", "../sysutils"]},
             [
-                'session.install("-e", "../testutils")',
-                'session.install("-e", "../sysutils")',
+                'session.install("-e", "../testutils", "-c", constraints_path)',
+                'session.install("-e", "../sysutils", "-c", constraints_path)',
             ],
         ),
         (
             {"unit_test_extras": ["abc", "def"]},
-            ['session.install("-e", ".[abc,def]")'],
+            ['session.install("-e", ".[abc,def]", "-c", constraints_path)'],
         ),
         (
             {"system_test_extras": ["abc", "def"]},
-            ['session.install("-e", ".[abc,def]")'],
+            ['session.install("-e", ".[abc,def]", "-c", constraints_path)'],
         ),
         (
             {"unit_test_extras_by_python": {"3.8": ["abc", "def"]}},
             [
                 'if session.python == "3.8":\n        extras = "[abc,def]"',
                 'else:\n        extras = ""',
-                'session.install("-e", f".{extras}")',
+                'session.install("-e", f".{extras}", "-c", constraints_path)',
             ],
         ),
         (
             {"system_test_extras_by_python": {"3.8": ["abc", "def"]}},
             [
                 'if session.python == "3.8":\n        extras = "[abc,def]"',
                 'else:\n        extras = ""',
-                'session.install("-e", f".{extras}")',
+                'session.install("-e", f".{extras}", "-c", constraints_path)',
             ],
         ),
         (
@@ -74,7 +74,7 @@
             [
                 'if session.python == "3.8":\n        extras = "[abc,def]"',
                 'else:\n        extras = "[tuv,wxyz]"',
-                'session.install("-e", f".{extras}")',
+                'session.install("-e", f".{extras}", "-c", constraints_path)',
             ],
         ),
         (
@@ -85,7 +85,7 @@
             [
                 'if session.python == "3.8":\n        extras = "[abc,def]"',
                 'else:\n        extras = "[tuv,wxyz]"',
-                'session.install("-e", f".{extras}")',
+                'session.install("-e", f".{extras}", "-c", constraints_path)',
             ],
         ),
     ],