Default the PathConverter (and descendants) to be non part isolating

[pgjones]

This is likely to be the expected for anything that is or extends the PathConverter and by adding it explicitly I hope it will cause less confusion, as seen in a recent issue 2783.

Note the PathConverter was already non part isolating due to the BaseConverter check.

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.