Temporarily get rid of the registration of Tcl_Finalize() as a

Temporarily get rid of the registration of Tcl_Finalize() as a
low-level Python exit handler.  This can attempt to call Python code
at a point that the interpreter and thread state have already been
destroyed, causing a Bus Error.  Given the intended use of
Py_AtExit(), I'm not convinced that it's a good idea to call it
earlier during Python's finalization sequence...  (Although this is
the only use for it in the entire distribution.)