1. Relying on `python3 -m pytest --help` to test for pytest can fail
because the pytest packages's __main__ is still invoked; this can
trigger import problems and falsely indicate that pytest is missing.
A simpler test is to just confirm that pytest is importable. If so,
the interpreter returns 0. Otherwise, an ImportError is thrown and
the interpreter will return 1.
2. Many templates require a custom do_check just to set PYTHONPATH to
either a build directory (especially for compiled extensions) or some
subdirectory of the source tree. Setting PYTHONPATH automatically to
the build directory should drastically reduce the need for custom
do_check in py3 templates. (This only applies to python3-module.sh
because pep517 builders will have unpredictable build directories.)
Closes: #31354.
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