Catch Py312+ warning when using ratarmount

[cphyc]

PR Summary

Since Python 3.12, os.fork() throws a deprecation warning when called from multiple threads.
This causes the tests relying on ratarmount (to mount archives) to fail. In this PR, I superficially address the issue by catching the warning on the test side.

The long-term solution would be to update ratarmount to a more recent version.

[cphyc]

@yt-fido test this please

[neutrinoceros]

We should probably not except the deprecation warning to pop up, as this will fail again if/when ratarmount fixes it in the future.
If the fix is just to ignore the warning for now, it should be done in conftest.py

[neutrinoceros]

I think we also need to report the problem upstream and link the issue in a comment next the the warning filter

[neutrinoceros]

this comment should be a condition. Something like
if find_spec("ratarmount") is not None and sys.version_info >= (3, 12):

[cphyc]

I have now caught the warning in the config file.

Note however that the warning is not a ratarmount issue but a yt one. It is caused by the fact that ratarmount is called in a subprocess (see comment: https://github.com/cphyc/yt/blob/3524c6b305fcd710aa0155dd30e9e4ae39085a5d/yt/loaders.py#L1792-L1795). In practice, this is only a problem if load_archive is called from a thread, which happens for pytests, but doesn't surface in most practical cases.

[neutrinoceros]

Dear me. Can't wait to see how this behaves in no-gil Python. Anyway, thank you for addressing this !