Use of `rmtree` causes `DeprecationWarning` on Python 3.12 alpha

[davidism]

The current Python 3.12 alpha has made a change to rmtree python/cpython#102828, deprecating the onerror parameter and replacing it with an onexc parameter. Something in Pytest's temp path fixtures calls an rm_rf function which calls rmtree with the onerror parameter.

pytest/src/_pytest/pathlib.py

Line 147 in 6dcd652

def rm_rf(path: Path) -> None:

When warnings are treated as errors, this makes pytest fail any test using temp paths.