no error when using subscripted `collections.abc.*` in runtime position when targeting python 3.8

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from collections.abc import Iterator

Iterator[int]

https://mypy-play.net/?mypy=basedmypy-2.3.0&python=3.8&gist=b28202ab91d513b652d8abca87ef02a8

This is because the defs in _collections_abc just import from typing

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another example:

from collections.abc import Iterator, cast

a = Iterator[int]