Improve stubgen tests

[bluenote10]

Improve test cases around #16486

This PR does not change any actual mypy behavior, only hardens the stubgen tests. The specific changes are:

• dedicated test cases: The existing pybind11 test cases originate from a pybind11 demo. They cover a specific topic involving geometry types and semi-implemented logic related to it. This somehow distracts from the aspects we are trying to test here from the mypy stubgen perspective, because it hides the actual intent of the bindings. I've simply started adding new test cases that clearly express via their name what the test case is addressing. I've kept the original demo stuff for now, so that the new cases are just an addition (overhead is negligible).
• running mypy on the generated stubs: In general it is crucial that the output produced by the stubgen can actually be type checked by mypy (this was the regression in Retain None (unreachable) when typemap is None with type(x) is Foo check #18486). This wasn't covered by the CI check so far. I've added check now, which would have avoided the regression. My goal for follow up PRs would be that we can use mypy --disallow-untyped-defs or even mypy --strict on the output.
• minor directory restructuring: So far the expected stub outputs were stored in folder names stubgen and stubgen-include-docs. This was a bit confusing, because the folder stubgen suggested it contains the stubgen (implementation). I went for expected_stubs_no_docs and expected_stubs_with_docs to make the role of the two folders more explicit.
• minor script bugfix: Fix a bug in test-stubgen.sh: The pre-delete functionality was broken, because the * was quoted and therefore did not match.

[JelleZijlstra]

I'm happy to merge this but would like @chadrik to also take a look (if you have time).

[chadrik]

This looks fantastic. My only nit is that it would be nice to change the name of the folder from pybind11_mypy_demo to pybind11_fixtures, since it's no longer a demo, it's an extension of the test suite.

That said, I have no problem merging as-is.

[bluenote10]

My only nit is that it would be nice to change the name of the folder from pybind11_mypy_demo to pybind11_fixtures, since it's no longer a demo, it's an extension of the test suite.

Absolutely, very good idea! I've just pushed that as well.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅