Add a `ScopeKind` for the `__class__` cell (#20048)
Summary
--
This PR aims to resolve (or help to resolve) #18442 and #19357 by
encoding the CPython semantics around the `__class__` cell in our
semantic model. Namely,
> `__class__` is an implicit closure reference created by the compiler
if any methods in a class body refer to either `__class__` or super.
from the Python
[docs](https://docs.python.org/3/reference/datamodel.html#creating-the-class-object).
As noted in the variant docs by @AlexWaygood, we don't fully model this
behavior, opting always to create the `__class__` cell binding in a new
`ScopeKind::DunderClassCell` around each method definition, without
checking if any method in the class body actually refers to `__class__`
or `super`.
As such, this PR fixes #18442 but not #19357.
Test Plan
--
Existing tests, plus the tests from #19783, which now pass without any
rule-specific code.
Note that we opted not to alter the behavior of F841 here because
flagging `__class__` in these cases still seems helpful. See the
discussion in
https://github.com/astral-sh/ruff/pull/20048#discussion_r2296252395 and
in the test comments for more information.
---------
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Co-authored-by: Mikko Leppänen <mleppan23@gmail.com>
