I played a bit with the patch and -v -Xshowrefcount.  The number of references and blocks left at exit varies (and is higher than for unpatched python).

It appears that a few (1-3) module dicts are not being purged because they have been "orphaned".  (i.e. the module object was garbaged collected before we check the weakref, but the module dict survived.)  Presumably it is the hash randomization causing the randomness.

Maybe 8 out of 50+ module dicts actually die a natural death by being garbage collected before they are purged.  Try

    ./python -v -Xshowrefcount check_purging.py