staticmethod return type is not inferred correctly

[lovasoa]

• Are you reporting a bug, or opening a feature request?
  reporting a bug
• 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:
    @staticmethod
    def f(x: int) -> str: raise NotImplementedError

class B(A):
    f = staticmethod(chr)

assert B().f(97) == "a"

• What is the actual behavior/output?

asd.py:6: error: Incompatible types in assignment (expression has type "staticmethod", base class "A" defined the type as "Callable[[int], str]")
asd.py:8: error: Cannot determine type of 'f'

• What is the behavior/output you expect?
  I think this should not raise any type error.

• What are the versions of mypy and Python you are using?
  mypy 0.720
  Python 3.7.5

[ilevkivskyi]

This is essentially a duplicate of #3482