Fix return value and type signature of shell_completion.add_completion_class function

[alexreg]

Decorators should return a possibly modfiied version of the original type. Hence, add_completion_class should return cls rather than merely None. This conforms to Python conventions, and allows type checking to work properly.

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.

[alexreg]

Pre-commit check failures seem to concern pre-existing issues.

[alexreg]

Hi @davidism. Is this okay for you?

[saroad2]

Hey @alexreg , thanks for the PR.

While this PR is pretty straight-forward, you didn't include any tests to verify it.
Can you add some tests to check that everything works smoothly?

[alexreg]

@saroad2 Sure. I added a couple of tests for the return value.

[saroad2]

Looks good.

I had one comment about the tests regarding the usage of _available_shells.clear(). If you have any questions about my comment, please feel free to ask.

[saroad2]

Hey @alexreg , thank you for responding to my review. I'm glad I was able to introduce you to new concepts.

I've added some small tweaks to your PR, mainly a small refactoring for the fixture, elaboration on the steps of the tests you wrote, and add one more test about using add_completion_class with the name parameter.

Happily merging the PR and hope to see your contributions again in the future!

[alexreg]

@saroad2 Looks good, re the tests. I'm not sure why I treated the dict like a value type in the fixture, but glad you fixed that too... Cheers.