+    Py_CLEAR(_datetimemodulestate(m)->PyDateTime_DateTimeType);
+    Py_CLEAR(_datetimemodulestate(m)->PyDateTime_DeltaType);
+    Py_CLEAR(_datetimemodulestate(m)->PyDateTime_TimeType);
+    Py_CLEAR(_datetimemodulestate(m)->PyDateTime_TimeZoneType);
+    Py_CLEAR(_datetimemodulestate(m)->PyDateTime_DateType);
+    Py_CLEAR(_datetimemodulestate(m)->PyDateTime_TZInfoType);

Style nit: I would really store the module state pointer in a variable here, instead of repeating _datetimemodulestate(m) every line.
(same in other places, such as the module init function)

+PyObject* _Get_State(struct PyModuleDef*);

I'm not sure why a module should define such generic a function, and especially not without a "Py" prefix.

Besides, review comments from issue15653 apply here.