fix: Prefix bootstrap file to prevent pytest reentrance (bazel-contrib#2230)

Pytest will, by default, discover tests in files named `test_*.py`. The
existing behaviour of the bootstrap setup is to create a file named e.g.
`test_foo_stage2_bootstrap.py` for a target named `test_foo`. This
causes pytest to reenter, leading to a crash.

To fix, prepend the generated stage2 file with an underscore so that
pytest doesn't match it.

---------

Co-authored-by: Richard Levasseur <[email protected]>

Author: mortenmj

CHANGELOG.md

@@ -57,6 +57,8 @@ A brief description of the categories of changes:
   ([#2186](https://github.com/bazelbuild/rules_python/issues/2186)).
 * (py_wheel) Fix incorrectly generated `Required-Dist` when specifying requirements with markers
   in extra_requires in py_wheel rule.
+* (rules) Prevent pytest from trying run the generated stage2
+  bootstrap .py file when using {obj}`--bootstrap_impl=script`
 
 
 ### Added

python/private/common/py_executable_bazel.bzl

@@ -330,7 +330,9 @@ def _create_stage2_bootstrap(
         imports,
         runtime_details):
     output = ctx.actions.declare_file(
-        "{}_stage2_bootstrap.py".format(output_prefix),
+        # Prepend with underscore to prevent pytest from trying to
+        # process the bootstrap for files starting with `test_`
+        "_{}_stage2_bootstrap.py".format(output_prefix),
         sibling = output_sibling,
     )
     runtime = runtime_details.effective_runtime