Built-in generics as super classes

[srittau]

Bug Report

Please consider the following stub file:

class Foo(list[str]):
    bar: list[str]

When checking this with mypy --python-version=3.8 foo.pyi (mypy 0.910, Python 3.9.4), I get the following output:

foo.pyi:1: error: "list" is not subscriptable
Found 1 error in 1 file (checked 1 source file)

Please note that it complains about line 1, but not line 2. It will not complain when running in Python 3.9+ mode. See also #9980 and #10303.