extensions: gracefully warn when doing min version check with no local version After doing a `make clean`, I started getting cryptic failures to import extensions with the `minimumhgversion` attribute on py3: *** failed to import extension evolve: '>' not supported between instances of 'int' and 'NoneType' *** failed to import extension topic: '>' not supported between instances of 'int' and 'NoneType' This now handles the `(None, None)` tuple before comparing, and disables the extension with the same friendly message as in py2. Differential Revision: https://phab.mercurial-scm.org/D9363