no error when subscripting `AbstractContextManager` when targeting python 3.8

[DetachHead]

from contextlib import AbstractContextManager

foo = AbstractContextManager[int] # runtime error, no mypy error

playground

[KotlinIsland]

• caused by: (🎁) Add a StubOnlyBase decorator for types that don't actually extend things, but do in the stubs (like Generic) typing#956

The easiest way to handle this situation would be to special case it.