Improve type hint

[pamelafox]

Thanks to a new test in main, I discovered that the return type of get_or_404 should not be Optional. I had chosen Optional since it can raise exceptions, but I've now learnt that Python typing does not document exception raising.

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.

[pamelafox]

@tonybaloney Can you review, since you reviewed the last one? One liner!

[tonybaloney]

LGTM 👍🏻 (I can't give an official approval on this repo)

[davidism]

@tonybaloney thanks for reviewing PRs! I sent you an invite so you can add official reviews 👍