Speed up cpdef method override checks also for Python subclasses with slots by...

Speed up cpdef method override checks also for Python subclasses with slots by assuming that slot attributes themselves never override type fields in the parent class (Python raises an error if you try).
See #2313.

Cython/Compiler/Nodes.py

@@ -4349,10 +4349,10 @@ class OverrideCheckNode(StatNode):
         # instance at a time.
         code.putln("static PY_UINT64_T tp_dict_version = 0, obj_dict_version = 0;")
         code.putln("if (likely("
-                   "Py_TYPE(%s)->tp_dictoffset && "
                    "Py_TYPE(%s)->tp_dict && "
                    "tp_dict_version == __PYX_GET_DICT_VERSION(Py_TYPE(%s)->tp_dict) && "
-                   "obj_dict_version == __PYX_GET_DICT_VERSION(_PyObject_GetDictPtr(%s))"
+                   "(!Py_TYPE(%s)->tp_dictoffset || "
+                   "obj_dict_version == __PYX_GET_DICT_VERSION(_PyObject_GetDictPtr(%s)))"
                    "));" % (
             self_arg, self_arg, self_arg, self_arg))
         code.putln("else {")