[stubgen] Add known return types to magic methods

[sobolevn]

This is based on the idea of https://github.com/JelleZijlstra/autotyping
Other magic methods can be added later.

CC @JelleZijlstra

[AlexWaygood]

You could maybe also add __hash__? If it's an actual method (hasn't been set to None), it's always going to return an int:

>>> class Foo:
...     def __hash__(self):
...         return 'bar'
... 
...     
>>> hash(Foo())
Traceback (most recent call last):
  File "C:\Users\alexw\AppData\Local\Programs\Python\Python310\lib\idlelib\run.py", line 578, in runcode
    exec(code, self.locals)
  File "<pyshell#15>", line 1, in <module>
TypeError: __hash__ method should return an integer

[sobolevn]

Sure, in the next PRs!

Right now it will require:

• __hash__: None vs def __hash__ tests
• Further work, quick test (I added "__hash__": "int" to my source code) shows that it does not work out of the box, probably it has some extra logic

[hauntsaninja]

Thanks, seems good!