closes bpo-36188: Clean up 'unbound' method left-overs. (GH-12169)

Methods are always bound, and `__self__` can no longer be `NULL`
(`method_new()` and `PyMethod_New()` both explicitly check for this).

Moreover, once a bound method is bound, it *stays* bound and won't be re-bound
to something else, so the section in the datamodel that talks about accessing
an methods in a different descriptor-binding context doesn't apply any more in
Python 3.