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Here's a new version (#6) that makes all interned strings
mortal unless explicitly requested with
PyString_InternImmortal(). There are no calls to that
function in the core.

I'm very tempted to check this in and see how it goes.

- Leave all the calls to PyString_InternInPlace(), since
that is still the recommended API.

- Got rid of the macro PyString_INTERN(), it was unused.

- Fixed the issue with getclassname() through an API change
(it's static so doesn't matter).

- Rewrote _Py_ReleaseInternedStrings(); it now simply clears
the immortality status, restores the stolen refcounts, and
then clears and decrefs the interned dict.