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
# name space package to host third party extensions
from __future__ import absolute_import
import pkgutil
__path__ = pkgutil.extend_path(__path__, __name__)