Fix "invalid escape sequence '\' (W605)"

Summary:
In python 3.6, using the \ char in a string that is not part of a
valid escape sequence will generate a Deprecation warning. For us it
means that flake8 is upset.

We use this in regex and in ascii art.

Reviewers: mmodell, Release-Engineering-Team

Reviewed By: mmodell, Release-Engineering-Team

Tags: Release-Engineering-Team

Differential Revision: https://phabricator.wikimedia.org/D1121