Cython will, in the right circumstances, offer a MethodType instance
where im_func is a builtin function. Any instance of MethodType is
automatically assumed to be a python-defined function (more
specifically, a function that has an inspectable signature), but
_set_signature was still conservative in its assumptions. As a result
_set_signature would return early with None instead of a mock since the
im_func had no inspectable signature. This causes problems deeper inside
mock, as _set_signature is assumed to _always_ return a mock, and
nothing checked its return value.

In similar corner cases, autospec will simply not check the spec of the
function, so _set_signature is amended to now return early with the
original, not-wrapped mock object.