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>

.flake8

@@ -7,6 +7,8 @@ ignore=
     E114,
     # E402: Module level import not at top of file
     E402,
+    # E731: do not assign a lambda expression, use a def
+    E731,
     # W503: Line break before binary operator
     W503,
     # W504: Line break after binary operator