python-support: adjust major versions

The plan previously documented was <=1.13.x is Python 2 and >=1.14.x
is Python 3.  Other projects that migrated Python versions and drop
support for older have tended to take a more drastic version jump to
make it clearer to users.  So lets adjust the plan to say <=1.x will
support Python 2, and >=2.x will be Python 3-only.

This also allows us to harmonize the repo launcher version.  It is
currently sitting at v1.26 and has been incremented independently of
the repo version for the life of the project.  While we might know
these lower nuances, pretty much no one else does and it just leads
to confusion: do I know version 1.26 or version 1.13.7?  Or do I
have both?  What does that even mean?

Once we update the major version to 2.0.0, we can also adjust the
launcher script to 2.0.0, and then the launcher release process will
be tied to a new repo release in general.

Bug: https://crbug.com/gerrit/10418
Change-Id: Idb2257371a06e56d2923cf717345c028f49176a2
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/240372
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>