Support six.add_metaclass

[gvanrossum]

Forked from #1764. I propose to give this less priority than six.with_metaclass, since the latter is more fundamental in some sense (it works for multiple bases with conflicting metaclasses where add_metaclass can't, see #1764 (comment)).

[gvanrossum]

There's also the issue that mypy accepted this, which doesn't work at runtime:

@six.add_metaclass(ArcMeta)
class Arc(Generic[T_co], Destroyable):

To address this we ought to duplicate Python's metaclass compatibility check.

[elazarg]

The consistency of the metaclass structure is already handled, once we tell mypy what add_metaclass means.