__class__ cell is defined for methods in py3

[Antony Lee]

On Py3,

class C:
    def ownclass(self):
        return __class__
print(C().ownclass())

executes correctly (`__class__` is the class in which the method was defined, which is not necessarily the type of `self`, see https://www.python.org/dev/peps/pep-3135/#specification). But pyflakes spuriously complains:

/tmp/foo.py:3: undefined name '__class__'