Fix __eq__ and __ne__.

[lukesneeringer]

This updates __eq__ to do type-checking correctly on all classes (except mock classes defined in unit test modules):

def __eq__(self, other):
    if not isinstance(other, type(self)):
        return NotImplemented

It also updates __ne__ to be exactly:

def __ne__(self, other):
    return not self == other

Fixes #3455.

[dhermes]

LGTM