Bug #128475: mimetools.encode (sometimes) fails when called from a thread.

pythonrun.c:  In Py_Finalize, don't reset the initialized flag until after
the exit funcs have run.
atexit.py:  in _run_exitfuncs, mutate the list of pending calls in a
threadsafe way.  This wasn't a contributor to bug 128475, it just burned
my eyeballs when looking at that bug.