stubgen: infer types for class attributes

[chadrik]

This PR dramatically improves the types generated for attributes that are instantiated within an __init__ function from typed arguments.

Here's an example input:

class Person:
    def __init__(self, name: str, age: int) -> None:
        self.name = name
        self.age = age

stubgen used to produce this output:

from _typeshed import Incomplete

class Person:
    name: Incomplete
    age: Incomplete
    def __init__(self, name: str, age: int) -> None: ...

Now it produces this output:

class Person:
    name: str
    age: int
    def __init__(self, name: str, age: int) -> None: ...

Pretty handy!

The key to this change is switching MypyOptions.semantic_analysis_only from True to False.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅