Consider replacing fake_filesystem with pyfakefs

[TheRealFalcon]

fake_filesystem is showing its age. It doesn't work for many (any?) pathlib related functions, there are still some older lower-level libraries it doesn't patch or can't easily patch, and there's no easy way to disable it in the middle of a test.

Meanwhile, pyfakefs has been actively maintained for years, already exists in most downstreams and supports more functionality, like patching the entire filesystem (including usage of pathlib), bypassing the patching, and pausing and resuming patching in the middle of a test.

I don't think migration would be hard as we could still use the fake_filesystem fixture as a shim, but instead just allow calling new methods on it.

Thoughts? Would upstream be open to me making this change?

[holmanb]

Thanks for the suggestion @TheRealFalcon. We are open to consider a contribution that uses this approach, especially if an initial implementation keeps the fake_filesystem as a shim layer to minimize the difference - this will make it easier for any remaining distros that don't have it packaged yet to carry a patch while they get it packaged. Ubuntu has had it in the archive since Jammy, so I don't think a patch there would be required. The project is maintained by pytest-dev, that also maintains pytest which we use already.

We would prefer to use stable features to keep compatibility across releases: targeting a maximum version of 4.5.4 or perhaps earlier. This of course means a bit greater risk in behavior differences due to spanning across major releases: but again keeping patch size small would help mitigate that risk in case a revert is needed on older distros.