Apply mypy-tests custom config to other mypy-based tests

[Avasam]

Extracted from #13627
Also relates to #12178 and #8403

We have the infra/tooling in place for per-stub mypy configuration. But those configs didn't extend to other tests, making it unusable since stubtest would still fail.

Easier with review with whitespace changes hidden.

[srittau]

Not a full review, but I'd prefer to keep the temporary file workaround logic separate from the actual logic for the temporary files.

[Avasam]

Yeah sure that makes sense. We may have to reuse temporary files elsewhere. It'd be nice if already separate.

The workaround used to only be done in tests/mypy_test.py, but I'll extract it and put it somewhere in ts_lib

[Avasam]

Maybe this should be a docstring ?

Suggested change

	# The configuration section in the metadata file looks like the following, with multiple module sections possible
	# [mypy-tests]
	# [mypy-tests.yaml]
	# module_name = "yaml"
	# [mypy-tests.yaml.values]
	# disallow_incomplete_defs = true
	# disallow_untyped_defs = true
	def mypy_configuration_from_distribution(distribution: str) -> list[MypyDistConf]:
	def mypy_configuration_from_distribution(distribution: str) -> list[MypyDistConf]:
	"""
	The configuration section in the metadata file looks like the following, with multiple module sections possible
	```toml
	[mypy-tests]
	[mypy-tests.yaml]
	module_name = "yaml"
	[mypy-tests.yaml.values]
	disallow_incomplete_defs = true
	disallow_untyped_defs = true
	```
	"""

[srittau]

I actually prefer this to be a comment. It doesn't really feel like something that's relevant to the API. That said, it would probably be better to document it in CONTRIBUTING instead.