Prepare for pytest 8

[lornajane]

Description of problem

Pytest 7 is out and shows some warnings about what will be deprecated in pytest 8. I'm running this with pytest 7.2.0 and python 3.10.7, and seeing the following:

rst2pdf/tests/conftest.py:428: 4400 warnings
  /home/lorna/projects/rst2pdf/rst2pdf/tests/conftest.py:428: PytestRemovedIn8Warning: The (fspath: py.path.local) argument to RstFile is deprecated. Please use the (path: pathlib.Path) argument instead.
  See https://docs.pytest.org/en/latest/deprecations.html#fspath-argument-for-node-constructors-replaced-with-pathlib-path
    return RstFile.from_parent(parent=parent, fspath=path)

rst2pdf/tests/conftest.py:430: 624 warnings
  /home/lorna/projects/rst2pdf/rst2pdf/tests/conftest.py:430: PytestRemovedIn8Warning: The (fspath: py.path.local) argument to SphinxFile is deprecated. Please use the (path: pathlib.Path) argument instead.
  See https://docs.pytest.org/en/latest/deprecations.html#fspath-argument-for-node-constructors-replaced-with-pathlib-path
    return SphinxFile.from_parent(parent=parent, fspath=path)

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

Looks like we need to update some of our path handling.