Infer descriptor type

Feature

For the common case of a descriptor which behaves like a normal attribute, infer:

class X:
    y: int = descriptor()

to:

class X:
    y: descriptor[int] = descriptor()

if descriptor is a Generic with one parameter. Extra checks should also be done to make sure that the descriptor is properly typed to behave like an attribute of that type (__get__ and __set__ expect that type on an instance).

Pitch

Type annotations apply to object instances by default, so it's strange to see y: descriptor[int] as type annotation while x.y returns an int.
You could also see a normal attribute as an invisible descriptor whose behavior is to set __dict__[name] (and raise AttributeError on the class), so this could be a way to unify these things.

It's also related to:
https://github.com/python/mypy/issues/7724
https://github.com/python/mypy/issues/5481

I implemented it partially as a plugin but only for my custom descriptors.

El issue no nombra archivos de implementación, pruebas ni un punto de entrada, así que primero revisa las discusiones relacionadas con los descriptores en los issues #7724 y #5481, además del comportamiento del plugin existente descrito aquí. Se consideraría completado cuando se haya decidido un enfoque para inferir el tipo de descriptor genérico y validar las firmas de get y set.