functools.partial no longer matches docs

[scop]

In mypy 0.710 due to I suppose commit e45f443, functools partial no longer corresponds with the docs, https://docs.python.org/3/library/functools.html#partial-objects

The docs state they're objects with func, args, and keywords, but they're no longer defined as such, thus cannot be annotated as partial, cannot access the documented attributes etc. For example, this resulted in no problem output with mypy 0.701:

from functools import partial
from typing import Callable

def foo(bar: Callable) -> None:
    func = bar
    while isinstance(func, partial):
        func = func.func

...but now with 0.710 gives:

t.py:6: error: Argument 2 to "isinstance" has incompatible type overloaded function; expected "Union[type, Tuple[Union[type, Tuple[Any, ...]], ...]]"
t.py:7: error: "Callable[..., Any]" has no attribute "func"

[JelleZijlstra]

#3077 will fix this.

[srittau]

Fixed.