version: fix running under Python 2

This gets the unittests passing again for now.

Change-Id: Ibed430a305bc26b907ad0ea424c7eec7de37e942
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/259994
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Jonathan Nieder <jrn@google.com>