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.

この issue では実装ファイル、テスト、エントリーポイントが示されていないため、まず issue #7724 と #5481 の descriptor 関連の議論と、ここで説明されている既存の plugin の動作を確認する。完了とするには、汎用 descriptor 型を推論し、その get と set のシグネチャを検証するための方針が決定されている必要がある。