stop testing Python 2.7

A recent change broke `repo version` on Python 2.7.  Rather than
fix it, lets drop Python 2.7 support so it can slowly rot.

Bug: https://crbug.com/gerrit/10418
Change-Id: I5c6e3d18e4a193b0a978062c23f7cea392e95d0f
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/259155
Reviewed-by: David Pursehouse <david.pursehouse@gmail.com>
Tested-by: Mike Frysinger <vapier@google.com>