* drop the unreasonable list invariant that ob_item should never come back

to NULL during the lifetime of the object.

* listobject.c nevertheless did not conform to the other invariants,
  either; fixed.

* listobject.c now uses list_clear() as the obvious internal way to clear
  a list, instead of abusing list_ass_slice() for that.  It makes it easier
  to enforce the invariant about ob_item == NULL.

* listsort() sets allocated to -1 during sort; any mutation will set it
  to a value >= 0, so it is a safe way to detect mutation.  A negative
  value for allocated does not cause a problem elsewhere currently.
  test_sort.py has a new test for this fix.

* listsort() leak: if items were added to the list during the sort, AND if
  these items had a __del__ that puts still more stuff into the list,
  then this more stuff (and the PyObject** array to hold them) were
  overridden at the end of listsort() and never released.

Include/listobject.h

@@ -30,9 +30,7 @@ typedef struct {
      *     0 <= ob_size <= allocated
      *     len(list) == ob_size
      *     ob_item == NULL implies ob_size == allocated == 0
-     *     If ob_item ever becomes non-NULL, it remains non-NULL for the
-     *         life of the list object.  The check for mutation in list.sort()
-     *         relies on this odd detail.
+     * list.sort() temporarily sets allocated to -1 to detect mutations.
      */
     int allocated;
 } PyListObject;