Mock can accept an spec object / class as argument, making sure
that accessing attributes that do not exist in the spec will cause an
AttributeError to be raised, but there is no guarantee that the spec's
methods signatures are respected in any way. This creates the possibility
to have faulty code with passing unittests and assertions.

Example:

from unittest import mock

class Something(object):
    def foo(self, a, b, c, d):
        pass

m = mock.Mock(spec=Something)
m.foo()

Adds the autospec argument to Mock, and its mock_add_spec method.

Passes the spec's attribute with the same name to the child mock (spec-ing
the child), if the mock's autospec is True.

Sets _mock_check_sig if the given spec is callable.

Adds unit tests to validate the fact that the autospec method signatures are
respected.