Cache types.GenericAlias in getitems

The "old-style" generic classes from the typing module (List, Dict, etc) cached typing._GenericAlias instances retuned from __getitem__:

>>> from typing import List
>>> List[int] is List[int]
True

But when this functionality was ported to built-in types, caching was not implemented:

>>> list[int] is list[int]
False

Since it is very common to subscript the same types with the same arguments, caching will be very beneficial here and will reduce memory footprint of heavily type-annotated modules.

Linked PRs

• gh-103541

首先檢查 typing 模組針對 getitem 的現有快取行為以及相關的 PR gh-103541。使用 list[int] is list[int] 範例確認預期結果；完成表示在不改變所述 typing 行為的情況下快取等價的內建泛型別名。