bpo-37337: Fix a GCC 9 warning in Objects/descrobject.c (pythonGH-14814)

Commit b1263d5 causes GCC 9.1.0 to
give a warning in Objects/descrobject.c.

Author: ZackerySpytz

Objects/descrobject.c

@@ -1047,7 +1047,7 @@ mappingproxy_copy(mappingproxyobject *pp, PyObject *Py_UNUSED(ignored))
             to the underlying mapping */
 
 static PyMethodDef mappingproxy_methods[] = {
-    {"get",       (PyCFunction)mappingproxy_get,        METH_FASTCALL,
+    {"get",       (PyCFunction)(void(*)(void))mappingproxy_get, METH_FASTCALL,
      PyDoc_STR("D.get(k[,d]) -> D[k] if k in D, else d."
                "  d defaults to None.")},
     {"keys",      (PyCFunction)mappingproxy_keys,       METH_NOARGS,