flake8: Suppress "E731 do not assign a lambda expression, use a def"

The Google Python Style Guide [1] says that lambdas are OK for
one-liners. All the current usages are one-liners, so let's just
suppress it.

[1] http://google.github.io/styleguide/pyguide.html#210-lambda-functions

Change-Id: I404c7a8e5e71870caf0f4604862cbf01db495863
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/255038
Tested-by: David Pursehouse <dpursehouse@collab.net>
Reviewed-by: Mike Frysinger <vapier@google.com>