Upgrade `typing.type_check_only` to be applicable to variables(and maybe import statements)

from typing import _KT, _promote

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Sure it has an underscore, but seems super sus that there is no mechanism to specify that a variable in a stub is just a type time helper and has no bearing on reality whatsoever.

You can specify explicitly that an import is accessible by importing it with an alias, although this in itself looks extremely hacky/workaroundy.

Also if it were possible to mark something in a stub as 'type time' only, then you could remove all the underscores and make a bunch of types look a lot nicer.

Very much related to https://github.com/python/mypy/issues/11503, where a module import in a stub can be completely fictitious, and there is currently no way to specify it one way or the other.

まず、変数と import 文に対して要求されている typing.type_check_only の動作を確認し、次にスタブ内の架空のモジュールインポートに関する関連 issue #11503 を読んでください。型チェック時のみ有効な望ましいセマンティクスとインポート動作が明確に仕様化され、合意されていれば完了です。