recognize staticmethod's __func__

To Reproduce

Run mypy against this code

class A(object):
    @staticmethod
    def open():
        return 123
    @staticmethod
    def proccess():
        return 456

    switch = {
        1: open.__func__,
        2: proccess.__func__,   
    }

p.s. this code snippet is copied from the first solution of https://stackoverflow.com/questions/41921255/staticmethod-object-is-not-callable

Expected Behavior

This code snippet should pass mypy's check.

Actual Behavior

s.py:10: error: "Callable[[], Any]" has no attribute "__func__"
s.py:11: error: "Callable[[], Any]" has no attribute "__func__"
Found 2 errors in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 0.910
• Mypy command-line flags: just file name
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.9.7
• Operating system and version: Debian rodete

Start by running the supplied staticmethod example with mypy and inspect how the type checker handles func access on the decorated methods. Trace the relevant staticmethod typing behavior and add or update a regression test for this snippet; done means the example passes mypy without the reported attribute errors.