Both `tmpdir` and `tmp_path` fixtures create temporary directories, but
the legacy `tmpdir` is based on third-party `py.path.local` objects
whereas `tmp_path` is based on the standard library `pathlib.Path`.

`py.path` provides classes for representing filesystem paths, but became
obsolete when `pathlib` was added to Python standard library. `pytest`
recommends creating temporary directories with the `tmp_path` fixture,
which uses `pathlib`, instead of the older `tmpdir` fixture, which uses
`py.path`. Furthermore, it is suggested to call `pytest` with
`-p no:legacypath` to remove support for `py.path` entirely, which helps
ensure `tmpdir` is not used at all. However, this also breaks any code
accessing `_pytest.nodes.Node.fspath`. Because `pytest-mpl` includes
such code then packages using it cannot turn off `py.path` support to
guard against `tmpdir` usage. Although replacing accessing `fspath` in
older versions of `pytest` is complicated, it is very simple since
`pytest` 7, so now at least the packages using recent versions of
`pytest` can choose to make use of the `-p no:legacypath` option.