Add mypy plugin support to stubtest configuration

[huynguyengl99]

Add mypy plugin support for stubtest

This PR adds support for mypy plugins in stubtest configuration, allowing packages that depend on plugins (like Django) to be properly validated. Implements mypy_plugins and mypy_plugins_config fields in METADATA.toml.

Fixes #13935

[huynguyengl99]

Hi @sobolevn and @srittau,

This PR adds support for mypy plugins in stubtest configuration, which enables packages like Django-based stubs to run stubtest correctly.

I've demonstrated this functionality in the recently created channels stubs package (PR #13949), which supports the well-known django-channels upstream package. The implementation successfully passes tests.

Key additions:

1. mypy_plugins field to specify mypy plugins
2. mypy_plugins_config field to provide plugin configuration (e.g., Django's django_settings_module)
3. Added support for django_settings.py in the @tests folder (by allowing this specific .py file where Python files are normally not permitted), providing a standard location for Django plugin configuration

I'd appreciate your feedback on this implementation and any suggestions for improvement. Thanks for your time!

[srittau]

Just one nit below. I think this is a useful addition to our test infrastructure, but I'll wait for other maintainer's opinions before merging.

[srittau]

Let's test the key also while we're here:

Suggested change

	return isinstance(obj, dict) and all(isinstance(item, dict) for item in obj.values())
	return isinstance(obj, dict) and all(isinstance(k, str) and isinstance(v, dict) for k, v in obj.items())

[huynguyengl99]

Affirmative, thank you

[sobolevn]

I think that supporting more complex stubs is great, if and only if other type checkers do not have any problems with that in the result.

The code looks good to me, thank you.