Adjust for the fact that Python 2.5 doesn't have collections.MutableSet.

When the MutableSet mixin isn't available, we fall back on just
deriving from object. This means that comparisons between membership
sets like __le__ as well as operators like __or__ aren't available,
but it seems unlikely those will be used for manipulating the AFS
protection database.

Signed-off-by: Evan Broder <broder@mit.edu>