Avoid deprecation warning in pytest

[hmaarrfk]

It seems that they don't like this anymore. It doesn't seem used.

[hmaarrfk]

Just FYI:

tests/conftest.py:13
  /home/mark/git/pylinalg/tests/conftest.py:13: PytestRemovedIn9Warning: The (startdir: py.path.local) argument is deprecated, please use (start_path: pathlib.Path)
  see https://docs.pytest.org/en/latest/deprecations.html#py-path-local-arguments-for-hooks-replaced-with-pathlib-path
    def pytest_report_header(config, start_path, startdir):

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html