Fix protocol’s __init__ behaviour to be same like in python >= 3.8

[dkraczkowski]

Hello everyone, I am author of a dependency injection container for python: https://pypi.org/project/kink/. Lately we run into some issues with older versions of python were we do use typing_extensions as a pollyfill for missing functionalities.

In python >= 3.8 _no_init function which is responsible for disabling Protocol's initialisation is defined in the global scope of a module, which allows us to do checks like here: https://github.com/kodemore/kink/blob/master/kink/inject.py#L75. Sadly in typing_extensions this function is defined as a local function so when doing the check memory address does not match and comparison fails. This is inconsistent with python >= 3.8 implementation and this PR fixes this inconsistency.

I have also prepared test to cover the use-case.

[the-knights-who-say-ni]

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[JelleZijlstra]

Thanks for your contribution!