[ty] __class_getitem__ is a classmethod (#20192)

## Summary

`__class_getitem__` is [implicitly a
classmethod](https://docs.python.org/3/reference/datamodel.html#object.__class_getitem__).

## Test Plan

Added regression test.

crates/ty_python_semantic/resources/mdtest/subscript/class.md

@@ -19,6 +19,12 @@ class Identity:
 reveal_type(Identity[0])  # revealed: str
 ```
 
+`__class_getitem__` is implicitly a classmethod, so it can be called like this:
+
+```py
+reveal_type(Identity.__class_getitem__(0))  # revealed: str
+```
+
 ## Class getitem union
 
 ```py