Py_Finalize(): disabled the second call of cyclic gc, and added extensive

comments about why both calls to cyclic gc here can cause problems.

I'll backport to 2.3 maint.  Since the calls were introduced in 2.3,
that will be the end of it.

Misc/NEWS

@@ -12,6 +12,16 @@ What's New in Python 2.4 alpha 1?
 Core and builtins
 -----------------
 
+- At Python shutdown time (Py_Finalize()), 2.3 called cyclic garbage
+  collection twice, both before and after tearing down modules.  The
+  call after tearing down modules has been disabled, because too much
+  of Python has been torn down then for __del__ methods and weakref
+  callbacks to execute sanely.  The most common symptom was a sequence
+  of uninformative messages on stderr when Python shut down, produced
+  by threads trying to raise exceptions, but unable to report the nature
+  of their problems because too much of the sys module had already been
+  destroyed.
+
 - Removed FutureWarnings related to hex/oct literals and conversions
   and left shifts.  (Thanks to Kalle Svensson for SF patch 849227.)
   This addresses most of the remaining semantic changes promised by