repo: drop Python 2 compat logic

Bug: 302871152
Change-Id: Ie7a0219e7ac582cd25c2bc5fb530e2c03bcbcc6e
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/390034


Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Commit-Queue: Mike Frysinger <vapier@google.com>
Tested-by: Mike Frysinger <vapier@google.com>