flake8: Suppress "F821 undefined name" inline for Python 2 names

All of the instances of this are related to Python 2 names that
don't exist in Python 3, and the warnings are raised when running
flake8 on Python 3.

All of these will go away once we completely remove support for
Python 2, so just suppress them inline. We don't globally suppress
the check so that we will still see legitimate errors if/when they
occur in new code.

Change-Id: Iccf955f50abfc9f83b371fc0af6cceb51037456f
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/255039
Tested-by: David Pursehouse <dpursehouse@collab.net>
Reviewed-by: Mike Frysinger <vapier@google.com>