bpo-38364: unwrap partialmethods just like we unwrap partials (#16600)

* bpo-38364: unwrap partialmethods just like we unwrap partials

The inspect.isgeneratorfunction, inspect.iscoroutinefunction and inspect.isasyncgenfunction already unwrap functools.partial objects, this patch adds support for partialmethod objects as well.

Also: Rename _partialmethod to __partialmethod__.
Since we're checking this attribute on arbitrary function-like objects,
we should use the namespace reserved for core Python.

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Co-authored-by: Petr Viktorin <encukou@gmail.com>