use simpler super() magic

Python 3 has a simpler super() style so switch to it to make the
code a little simpler and to stop pylint warnings.

Change-Id: I1b3ccf57ae968d56a9a0bcfc1258fbd8bfa3afee
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/297383


Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>