Was running into it, so just added a quick fix that should hopefully
get us through until rudi gets in his real fix.

This commit just delays freeing the class objects until the end of the
sweep.  It doesn't deal with a number of corner cases such as if
a class and a metaclass get freed at the same time (they both get delayed
to the same point), or if we free a class with weakrefs to it (it just
gets treated like a normal weakreferent).