flake8: Ignore 'line break before/after binary operator'

- W503 line break before binary operator
- W504 line break after binary operator

There doesn't seem to be a nice way of fixing all of these without
replacing W503 with W504 or vice-versa, or unwrapping them resulting
in excessively long lines. Let's just suppress them.

Change-Id: I7846d0124054f58e1cb480d4459cd9c86b737a50
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254608
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: David Pursehouse <dpursehouse@collab.net>