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.

Beginne damit, das angeforderte Verhalten von typing.type_check_only für Variablen und Importanweisungen zu überprüfen, und lies anschließend den zugehörigen Issue #11503 über fingierte Modulimporte in Stubs. Als abgeschlossen gilt die Aufgabe, wenn die gewünschte Semantik, die nur zur Typprüfungszeit gilt, und das Importverhalten klar spezifiziert und abgestimmt sind.