Specify Optional parameters in Graph.triples_choices

[slahn]

Summary of changes

Change the typing of Graph.triples_choices and Store.triples_choises
to match the actual types allowed by the code.

The two non-list parameters can be None, but this is not reflected in
the type hint today.

Introduce a type alias to simplify method signatures, and update all
overloads of triples_choises to use this alias.

Checklist

• [✓] Checked that there aren't other open pull requests for
  the same change.
• [✓] Checked that all tests and type checking passes.
  • Did not run webtests (pytest -m "not webtest"), since I could not get them working at all.
    7266 passed, 61 skipped, 333 deselected, 330 xfailed, 36 xpassed, 6925 warnings
• [✓] Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[ashleysommer]

@slahn Note, the main branch is the v7.x maintenance branch, so I'm not sure if this change is required in main.
Can you check if this same change needs to be done to v8 branch, where the latest work is?

[edmondchuc]

pre-commit.ci run