Let's try a different technique.

NetscapeSPKI seems like it's robust against version changes.  Kinda
grasping at straws here, so we'll see what the buildbot makes of it.

Also document the properties we're at least ''trying'' to provide here,
and provide a test for one of those properties.

git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/branches/inconsistent-hash-7651@43155 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb