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.