Update to using pytest's tmp_path over legacy tmpdir

[matthewfeickert]

Nothing breaks yet, but pytest now uses the tmp_path and tmp_path_factory fixtures over the (now "legacy") tmpdir and tmpdir_factory fixtures.

From the pytest docs (https://docs.pytest.org/en/7.4.x/how-to/tmp_path.html#the-tmpdir-and-tmpdir-factory-fixtures):

The tmpdir and tmpdir_factory fixtures are similar to tmp_path and tmp_path_factory, but use/return legacy py.path.local objects rather than standard pathlib.Path objects.

Note

These days, it is preferred to use tmp_path and tmp_path_factory.

In order to help modernize old code bases, one can run pytest with the legacypath plugin disabled:

pytest -p no:legacypath

This will trigger errors on tests using the legacy paths. It can also be permanently set as part of the addopts parameter in the config file.

While this should be a small internal change, it seems like still using tmpdir is keeping projects that use pytest-mpl form using the legacypath plugin to do validation (c.f. scikit-hep/pyhf#2384 (comment)).