[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.

Author: sharkdp

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

crates/ty_python_semantic/src/types/function.rs

@@ -725,7 +725,10 @@ impl<'db> FunctionType<'db> {
     /// classmethod.
     pub(crate) fn is_classmethod(self, db: &'db dyn Db) -> bool {
         self.has_known_decorator(db, FunctionDecorators::CLASSMETHOD)
-            || self.name(db) == "__init_subclass__"
+            || matches!(
+                self.name(db).as_str(),
+                "__init_subclass__" | "__class_getitem__"
+            )
     }
 
     /// If the implementation of this function is deprecated, returns the `@warnings.deprecated`.

crates/ty_python_semantic/src/types/infer.rs

@@ -9203,7 +9203,7 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                         }
                     }
 
-                    match ty.try_call(db, &CallArguments::positional([value_ty, slice_ty])) {
+                    match ty.try_call(db, &CallArguments::positional([slice_ty])) {
                         Ok(bindings) => return bindings.return_type(db),
                         Err(CallError(_, bindings)) => {
                             if let Some(builder) =