Use weakrefs to hold onto classes #2521.
This also causes the _weakref module to be built into the core.
diff --git a/Modules/Setup.dist b/Modules/Setup.dist
index e76e0df..e2b0c59 100644
--- a/Modules/Setup.dist
+++ b/Modules/Setup.dist
@@ -118,6 +118,7 @@
# if $HOME is not set
_sre _sre.c # Fredrik Lundh's new regular expressions
_codecs _codecsmodule.c # access to the builtin codecs and codec registry
+_weakref _weakref.c # weak references
# The zipimport module is always imported at startup. Having it as a
# builtin module avoids some bootstrapping problems and reduces overhead.