[Python-Dev] Review request: optimizing typing ABC caches

[Ivan Levkivskyi]

There is a PR implementing typing ABC cache optimization:
https://github.com/python/typing/pull/383

The main idea is straightforward: subscripted generic ABCs like
Iterable[int], Iterable[str], etc. should not have separate ABC caches
(positive and negative), since they all are equivalent to plain Iterable at
runtime.

It is proposed that they will share their caches with a parent ABC from abc
module for abstract collections, or with original (unsubscripted) class
generic for concrete classes.

Inada-san confirmed that this optimization reduces the memory footprint.
I will be grateful for a code review.

Best regards,
Ivan
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