Manually defining a class via attributes does not allow for passing kwargs

[stuhood]

The recommended method of creating a subclass from https://github.com/pybind/pybind11/issues/1193#issuecomment-429451094 does not seem to allow for passing keyword arguments.

This test fails with:

    def test_kw_only():
>       assert (m.KwOnlyMethod().test("x", y="y") == 0)
E       TypeError: (): incompatible function arguments. The following argument types are supported:
E           1. (x: object, *, y: object, arg1: object) -> int
E
E       Invoked with: <importlib._bootstrap.MwOnlyMethod object at 0x11029e900>, 'x'; kwargs: y='y'


../../../../tests/test_class.py:507: TypeError