Support GenericMeta

[gvanrossum]

This forked off #1764. Example:

from typing import *
from typing import GenericMeta  # needed for python3 runtime
import six

class _DestroyableMeta(type): pass

class Destroyable(six.with_metaclass(_DestroyableMeta)):
    pass

T_co = TypeVar('T_co', bound='Destroyable', covariant=True)

class ArcMeta(GenericMeta, _DestroyableMeta):
    pass

class Arc(six.with_metaclass(ArcMeta, Generic[T_co], Destroyable)):
    pass

Even with #3364 this still gives an error on the occurrence of Generic in the base classes:

u.py:16: error: Invalid type "typing.Generic"
u.py:16: error: Invalid base class

[ilevkivskyi]

Should anything be done here after #3364 is merged?
(It looks like we already have #3365 to track possible metaclass conflicts.)

[gvanrossum]

Yeah, this example now works.