repo: reexec into Python 3 under Windows

Hopefully enough issues should be resolved now that we can start
forcing Windows users into Python 3 too.

Change-Id: Ic4aad6a0b35ffec7d1372e3da6fca11a2b6fde0b
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/255353
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>