Direct import for metaclass ABCMeta

[elazarg]

Added from abc import ABCMeta in importlib.

This change is needed for mypy/#2365.

[gvanrossum]

That is very strange. Why is that needed? If you don't know, I suggest you research it further before answering. :-)

[elazarg]

I can't say I know for sure, but I have a strong suspicion. I believe this is due to the priority given to from imports inside mypy. If so, this PR is an implementation detail of mypy leaking into typeshed, but I think the reason behind it - the fact that the name is immediately needed, IIUC - is relevant to metaclasses.

Perhaps this can be changed from inside mypy; if so, I can put this PR on hold. But the solutions I came up with were ugly hacks.

[gvanrossum]

In this case I'd say add abstractmethod too and get rid of the previous line (import abc), and modify all @abc.abstractmethod to @abstractmethod.

[gvanrossum]

OK, I think that is actually reasonable. A tweak just in mypy might be to detect that a class or other object in an imported module is used in a position where it is immediately needed (e.g. base class, metaclass, decorator) but that feels fairly hard since all those calculations have to be done in semanal.FirstPass which doesn't have a lot of understanding yet.