extensions: prevent a crash on py3 when testing a bad extension minimum A `None` placeholder is populated for each missing component by `util.versiontuple()`, which could safely be used with `>` on py2, but not py3. I guess there's another hole here where if the string is entirely bogus (i.e no numbers), it will be treated as 0.0, and always load. But that's always been the case. Differential Revision: https://phab.mercurial-scm.org/D11475