Corrected n3 method of path objects (fixes #2503)

[lukasmu]

Summary of changes

This is an attempt to fix #2503. Previously, the n3 method of path objects returned unexpected/invalid strings when the path objects were non-trivial (i.e. nested, consisting of property path groups). Examples can be found in the related issue and the tests.

This change fixes a bug. It might impact on users that previously relied on the invalid implementation of the n3 method. Not sure if/where this should be added to the documentation.

• Fixes N3 method for path objects is bugged #2503

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• If the change has a potential impact on users of this project:
  • Added or updated tests that fail without the change.
  • Updated relevant documentation to avoid inaccuracies.
  • Considered adding additional documentation.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[coveralls]

coverage: 90.926%. first build when pulling 70f1d90 on lukasmu:bugfix/path_n3_method into bdd574e on RDFLib:main.

[aucampia]

Thanks for the pull request @lukasmu - it looks good to me. I added some additional tests that all passed, and a return type annotation.