Disable assertion rewriting of external modules for `python_files = *.py`

[lucascolley]

#10845 (comment)

i think that part deserves a new issue that specifically handles that part

cc @RonnyPfannschmidt @The-Compiler

[The-Compiler]

For context, from #10845 (comment):

• pytest does assertion rewriting for everything (even third-party modules outside of the test rootdir) if python_files = *.py is set. IMHO that is a pytest bug, though I never really was deeply involved with the assertion rewriting code, so I'd love to hear what other maintainers think.