Annotate Serializer.serialize and descendants

[ajnelson-nist]

This patch aligns the type signatures on Serializer subclasses, including renaming the arbitrary-keywords dictionary to always be **kwargs. This is in part to prepare for the possibility of adding *args as a positional-argument delimiter.

References:

• Sorting Turtle output? #1890 (comment)

Summary of changes

Checklist

• Checked that there aren't other open pull requests for
  the same change.
  • Note: This might bump into PR 2115, but I don't think I see any files that both PRs touched.
• Checked that all tests and type checking passes.
• If the change adds new features or changes the RDFLib public API:
  • Created an issue to discuss the change and get in-principle agreement. (Skipped - supplements Issue Sorting Turtle output? #1890)
  • Considered adding an example in ./examples. (Skipped - supplements Issue Sorting Turtle output? #1890)
• If the change has a potential impact on users of this project: (Skipped - believed N/A)
  • 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.

[ajnelson-nist]

This PR is ready for review and merge.

I'll be folding it into #1978 too.

[coveralls]

coverage: 90.249% (+0.001%) from 90.248%
when pulling 2dfdfc0 on ajnelson-nist:issue-1890-serializer-types
into 086a4f7 on RDFLib:main.

[ashleysommer]

Nice one. Thanks.