[Feature request / documentation enhancement proposal if it turns out this exists : ]

Hello, I'm looking for a way to support code like the following:

class Color:
    ''' [Many useful methods here] '''

class Palette:
    def __setattr__(self, key: str, value: object) -> None:
        ''' [Implicitly convert `value` to a `Color`.] '''

class SoftPastels(Palette):
    green: Color
    blue: Color

sp = SoftPastels()
sp.green = (127, 255, 127)  # -- "Incompatible types in assignment", MyPy 0.701
sp.blue = '#8888ff'         # -- "Incompatible types in assignment", MyPy 0.701

My use-case involves implicitly converting to numpy.ndarrays and/or h5py.Datasets, and I've seen this pattern (performing implicit conversion on attribute assignment) used to improve API ergonomics in a handful of other libraries (e.g. Pandas and the Blender API, IIRC).

Is there (or could there be) any way to type this?