Add type annotations to function signatures in credential_plugins_test and importable_test modules

[djyasin]

This PR contains the type annotations to function signatures for credential_plugins_test.py and tests/importable_test.py

[webknjaz]

Judging by the RTD failures, the branch needs to be rebased (and some CI jobs will become green as a result).

[webknjaz]

@djyasin here's the current state of MyPy coverage: https://app.codecov.io/gh/ansible/awx-plugins?flags%5B0%5D=MyPy. You can look into that or check what's in python3 -Im webbrowser $(ls -1 .tox/.tmp/.test-results/mypy--py-*/index.html | head -n 1) after each time you re-run tox r -e pre-commit -q -- mypy --all-files locally.

[djyasin]

This PR contains all of the fixes for credential_plugins_test.py and tests/importable_test.py

[webknjaz]

This problem https://github.com/ansible/awx-plugins/actions/runs/10740379607/job/29788419764?pr=26#step:18:29 shows that there is a dependency on awx:

src/awx_plugins/credentials/plugins.py:7:1: error: Cannot find implementation
or library stub for module named "awx.main.models.credential" 
[import-not-found]

Though, we cannot physically declare it due to it not being on PyPI (well, there's a wrong awx there but not ours). So we have no choice but to ignore it since we can't check against it until we decouple the two projects, exterminating the circular dependencies between them.

The way to do it is by reproducing a section like https://github.com/sphinx-contrib/sphinxcontrib-towncrier/blob/b01f3b0/.mypy.ini#L36-L40 and configuring just that section to pretend there are no such imports...

[webknjaz]

FYI, this failure https://github.com/ansible/awx-plugins/actions/runs/10818045643/job/30012894850?pr=26#step:18:49 is flaky. I reported it upstream via sphinx-contrib/spelling#227. For now, just restart the job if you see this error.

[webknjaz]

Let's move the unrelated fixes elsewhere so that this PR won't have to accumulate more changes required for those unrelated fixes to be complete.

[webknjaz]

@djyasin I just realized that the PR description and title need to be updated to reflect what's fixed. And also to be accurate...

[webknjaz]

There's a few places that need to be addressed still.

[webknjaz]

@djyasin feel free to squash the commits and put this into the merge queue.

[webknjaz]

@djyasin I think the PR title needs to be clarified still. It's pretty generic in describing what is done. It would be nice to see what exactly is being changed.

Also, some MyPy violations going away is one of the effects of the patch. The title should be talking about the change that leads to those effects. Like adding typing. Or rather adding type annotations to function signatures in these modules. Not sure if this covers all violations in these modules (probably not?)