Subclasses of string can no longer be interned. The semantics of

Subclasses of string can no longer be interned.  The semantics of
interning were not clear here -- a subclass could be mutable, for
example -- and had bugs.  Explicitly interning a subclass of string
via intern() will raise a TypeError.  Internal operations that attempt
to intern a string subclass will have no effect.

Added a few tests to test_builtin that includes the old buggy code and
verifies that calls like PyObject_SetAttr() don't fail.  Perhaps these
tests should have gone in test_string.