Allow covariance for read-only attributes

[TH3CHARLie]

resolves #5836

For now, I've not added any tests yet. I've tested against code samples in #5836 and #3260 and the patch is working but adding either of them seems to require modifying a lot in test fixtures

The new helper function is_writable_attribute is actually borrowed from check_no_writable. I'd also like to refactor the latter function's name and implement it using the new helper in this PR, since I think its name is kind of misleading considering the comments

mypy/mypy/checker.py

Lines 2391 to 2402 in ea3c65c

	def check_no_writable(self, name: str, base_node: Optional[Node], ctx: Context) -> None:
	"""Check that a final variable doesn't override writeable attribute.
	This is done to prevent situations like this:
	class C:
	attr = 1
	class D(C):
	attr: Final = 2
	x: C = D()
	x.attr = 3 # Oops!
	"""

[JukkaL]

but adding either of them seems to require modifying a lot in test fixtures

What's missing in text fixtures? Maybe you can simplify the examples to not rely on so many features.

[TH3CHARLie]

but adding either of them seems to require modifying a lot in test fixtures

What's missing in text fixtures? Maybe you can simplify the examples to not rely on so many features.

Good point. I've simplified the case from #5836 and added it. I've also done some refactor work to reuse the new helper added and modify the function's name to make it less misleading. But it is not the main focus of the PR and I'd like to know your thoughts on it. @JukkaL

[msullivan]

OK this seems like a reasonable improvement. I think this doesn't affect all attributes. Regular attributes we just always allow covariant overriding of, even when it is totally wrong.