untrackable objects are not tracked by the garbage collector. This can
reduce the size of collections and therefore the garbage collection overhead
on long-running programs, depending on their particular use of datatypes.
(trivia: this makes the "binary_trees" benchmark from the Computer Language
Shootout 40% faster)
test_tuple.py and test_list.py. Common tests for tuple, list and UserList
are shared (in seq_tests.py and list_tests.py). Port tests to PyUnit.
(From SF patch #736962)