[3.7] Fix misleading mentions of tp_size in comments. (GH-9136)

Many type object initializations labeled a field "tp_size" in the
comment, but the name of that field is tp_basicsize..
(cherry picked from commit 0e0bc4e221f592f305d335faf5f8046484eb9238)

Co-authored-by: Peter Eisentraut <peter@eisentraut.org>
diff --git a/Objects/namespaceobject.c b/Objects/namespaceobject.c
index e5698e6..7de102e 100644
--- a/Objects/namespaceobject.c
+++ b/Objects/namespaceobject.c
@@ -201,7 +201,7 @@
 PyTypeObject _PyNamespace_Type = {
     PyVarObject_HEAD_INIT(&PyType_Type, 0)
     "types.SimpleNamespace",                    /* tp_name */
-    sizeof(_PyNamespaceObject),                 /* tp_size */
+    sizeof(_PyNamespaceObject),                 /* tp_basicsize */
     0,                                          /* tp_itemsize */
     (destructor)namespace_dealloc,              /* tp_dealloc */
     0,                                          /* tp_print */