stubgen infers function returns None if it passes or raise NotImplementedError

• Are you reporting a bug, or opening a feature request?
  Reporting a bug, although I'm not sure what the intended behavior is.

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

class A:
    def f(self):
        raise NotImplementedError

    def g(self):
        """A is not an abstract base class"""
        return "Hello World"

class B(A):
    def f(self):
        return 5

class C:
    def f(self):
        pass

• What is the actual behavior/output?
  stubgen output:

class A:
    def f(self) -> None: ...
    def g(self): ...

class B(A):
    def f(self): ...

class C:
    def f(self) -> None: ...

• What is the behavior/output you expect?

class A:
    def f(self): ...
    def g(self): ...

class B(A):
    def f(self): ...

class C:
    def f(self): ...

This is because I would like to write code like

def h(a: A):
    return a.f()

h(B())

without typing errors. In realistic use cases, A has multiple subclasses.

• What are the versions of mypy and Python you are using?
  Python 3.7.7
  mypy 0.782
  Do you see the same issue after installing mypy from Git master?
  Yes

Start with the stubgen behavior shown in the issue and reproduce it using the A, B, and C example. Trace how stubgen infers return annotations for methods that pass or raise NotImplementedError, then verify the generated stubs no longer add None in those cases while preserving the shown subclass methods.