mypy does not handle __class_getitem__

PEP 560 defines __class_getitem__ as a way to enable indexing a class (to define custom generic types for instance), but mypy does not seem to recognize this.

To Reproduce

$ cat test.py
class A:
    def __class_getitem__(cls, item):
        return cls

x = A[int]

$ mypy test.py

Expected Behavior

Success: no issues found in 1 source file

Actual Behavior

test.py:6: error: "A" expects no type arguments, but 1 given
Found 2 errors in 1 file (checked 1 source file)

Environment

• mypy 0.910
• Python 3.9.7
• Darwin arm64

Start by running the reported test.py example with mypy and confirm the errors around A[int]. Trace the type-checking path for class_getitem and add coverage for this case; done means the example reports no issues as expected.