chore: restrict rsa version to <=4.0 for Python 2.7 compatibility (#135)

* chore: restrict `rsa` version to <=4.0 for Python 2.7 compatibility

* Restrict RSA dependency version for Python 2 only

Author: plamut

setup.py

@@ -36,6 +36,10 @@
     "google-resumable-media >= 0.5.0, < 0.6dev",
     "protobuf >= 3.6.0",
     "six >=1.13.0,< 2.0.0dev",
+    # rsa >= 4.1 is not compatible with Python 2
+    # https://github.com/sybrenstuvel/python-rsa/issues/152
+    'rsa <4.1; python_version < "3"',
+    'rsa >=3.1.4, <5; python_version >= "3"',
 ]
 extras = {
     "bqstorage": [