complex_coerce(): add explicit PyComplex_Check() test. Previously,

complex_coerce() would never be called with a complex argument,
because PyNumber_Coerce[Ex] doesn't bother calling the type's coercion
method if the values already have the same type.  But now, of course,
it's possible to pass an instance of a complex *subtype*, and those
must be accepted.

Objects/complexobject.c

@@ -538,6 +538,11 @@ complex_coerce(PyObject **pv, PyObject **pw)
 		Py_INCREF(*pv);
 		return 0;
 	}
+	else if (PyComplex_Check(*pw)) {
+		Py_INCREF(*pv);
+		Py_INCREF(*pw);
+		return 0;
+	}
 	return 1; /* Can't do it */
 }