Warn that rmdir() will stop removing non-empty directories

[itzzdev09]

Towards #559.

UPath.rmdir() removes a non-empty directory and everything in it, while PosixUPath/WindowsUPath and a wrapped pathlib.Path raise OSError, because they go through pathlib.Path.rmdir(). Same call, same local directory, two outcomes depending on how the path was spelled:

UPath("test").rmdir()          # OSError: Directory not empty
UPath("file://test").rmdir()   # removes test/ and its contents

As suggested in the issue, this does not change behaviour yet. recursive now defaults to UNSET_DEFAULT, and when it is not given and the directory is not empty, rmdir() warns:

UPath.rmdir() currently removes a non-empty directory and its contents. In universal-pathlib 0.4.0 it will raise, like pathlib.Path.rmdir() does. Pass recursive=True to keep removing the contents.

Passing recursive=True or recursive=False explicitly is unchanged and silent, so callers can opt out of the warning now and keep working after the default flips. The pathlib-backed classes already behave the way 0.4.0 will, so they do not warn.

One small fix comes along: the emptiness check was next(self.iterdir()), which raised StopIteration on an empty directory instead of falling through. It is now next(self.iterdir(), None).

Tests

test_rmdir_not_empty_warns_about_the_future_default and test_rmdir_not_empty_recursive_does_not_warn in the shared BaseTests, so every implementation is covered, with an override in TestProxyPathlibPath where the wrapped pathlib.Path raises instead.

Local suite: 2443 passed. test_pathlib_backport.py::test_signature_glob_pathlike[memory] fails on main here too, and the hdfs errors are a missing optional dependency.

Not in this PR

rmdir(recursive=False) on an empty directory calls fs.rm(..., recursive=False), which fsspec's LocalFileSystem rejects with ValueError: Cannot delete directory, set recursive=True. That is separate from the default change, so I left it alone; happy to follow up if you want it fixed before 0.4.0, since it becomes the common path once the default flips.