SF bug 1061968: threads: segfault or Py_FatalError at exit

SF bug 1061968: threads: segfault or Py_FatalError at exit

PyGILState_Ensure():  The fix in 2.4a3 for bug 1010677 reintroduced thread
shutdown race bug 225673.  Repaired by (once again) ensuring the GIL is
held whenever deleting a thread state.

Alas, there's no useful test case for this shy bug.  Four years ago, only
Guido could provoke it, on his box, and today only Armin can provoke it
on his box.  I've never been able to provoke it (but not for lack of
trying!).

This is a critical fix for 2.3.5 too, since the fix for 1010677 got
backported there already and so also reintroduced 225673.  I don't intend to
backport this fix.  For whoever (if anyone) does, there are other thread
fixes in 2.4 that need backporting too, and I bet they need to happen first
for this patch to apply cleanly.