Check keyword arguments in class definitions

[JelleZijlstra]

Currently, this incorrect code passes mypy:

class C(a=3):
    pass

It should check that __new__/__init_subclass__ accepts these keyword arguments.

This doesn't seem high-priority to fix since custom class kwargs are rarely used and this should usually cause import-time errors anyway when running the code, but I'm reporting it so it can be tracked.

[ilevkivskyi]

Support for checking against __init_subclass__() was added in 0.730. Checking against __new__() of metaclass is still to do (btw it makes me think this can cause some false positives).