bpo-38823: Fix compiler warning in _ctypes on Windows (pythonGH-23258)

Explicitly cast PyExc_Exception to PyTypeObject* to fix the warning:

    modules\_ctypes\_ctypes.c(5748): warning C4133: '=':
    incompatible types - from 'PyObject *' to '_typeobject *'

Author: vstinner

Modules/_ctypes/_ctypes.c

@@ -5745,7 +5745,7 @@ _ctypes_add_types(PyObject *mod)
     TYPE_READY(&StructParam_Type);
 
 #ifdef MS_WIN32
-    TYPE_READY_BASE(&PyComError_Type, PyExc_Exception);
+    TYPE_READY_BASE(&PyComError_Type, (PyTypeObject*)PyExc_Exception);
 #endif
 
 #undef TYPE_READY