Use typeshed for builtins rather than the scraper #533

This will allow better understanding of builtins since scraper does not always generate correct result and for return type we have to do specialization. Ex, in 3.7 scraper generates pow(x, y, z) rather than pow(x, y, z = None) and specialization just returns type of x. Typeshed stub is much more detailed, but extensively uses typing module so understanding of typing has to be improved as well.

@overload
def pow(x: int, y: int) -> Any: ...  # The return type can be int or float, depending on y.
@overload
def pow(x: int, y: int, z: int) -> Any: ...
@overload
def pow(x: float, y: float) -> float: ...
@overload
def pow(x: float, y: float, z: float) -> float: ...