TestPyCacheDir.test_sys_pycache_prefix_integration fails on Windows / Python 3.9

[The-Compiler]

See https://github.com/pytest-dev/pytest/actions/runs/1448359677 and https://github.com/pytest-dev/pytest/actions/runs/1449499760:

        # test file: rewritten, custom pytest cache tag
        test_foo_pyc = get_cache_dir(test_foo) / ("test_foo" + PYC_TAIL)
>       assert test_foo_pyc.is_file()
E       AssertionError: assert False
E        +  where False = <bound method Path.is_file of WindowsPath('C:/Users/runneradmin/AppData/Local/Temp/pytest-of-runneradmin/pytest-0/pope...n/pytest-0/popen-gw0/test_sys_pycache_prefix_integration0/src/test_foo.cpython-39-pytest-7.0.0.dev312+g17b38259f.pyc')>()

#9294 was a doc-fix only though... Test introduced in #5864 for #4730.

[nicoddemus]

Hmm unfortunately I cannot reproduce it locally, it works for me on Python 3.9.8...

Perhaps we should mark it as xfail for now? Better than having this breaking on CI.

[RonnyPfannschmidt]

output indicates a bug in get_cache_dir

i don't see a __pycache__ in the filename

[RonnyPfannschmidt]

@The-Compiler @nicoddemus i realized the issue - the test monkeypatche sys instead of the enviroment variables, which breaks for subprocess based running

[RonnyPfannschmidt]

my hypothesis turned out wrong