Subclasses of typing.Sequence (&c) pretend to be superclasses of all registered members of abc.Sequence (&c)

Check this out yo:

In [1]: from typing import Sequence
In [2]: class DumbSeq(Sequence): pass
In [3]: issubclass(tuple, DumbSeq)
Out[3]: True

This has to do with the implementation of GenericMeta, where our __subclasscheck__ ends with this line:

        return issubclass(cls, self.__extra__)

And for the container classes in typing, they're defined to have the collections.abc classes as their __extra__.

Pondering a fix to suggest.

Before you ponder much more, maybe the only open PR addresses this?
#207

Yup looks like.

Also, this looks similar: #202

Also correct. I'd close this as a duplicate.

In particular, you were apparently testing with an outdated version of typing.py. :-(

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