Minor cleanup for flake8

[dannysauer]

Minor changes to fix flake8 warnings

• use raw strings in regex methods
• inline-suppress an incorrect unused variable warning
• update to flake8 version 3.6.0 (release notes: http://flake8.pycqa.org/en/latest/release-notes/3.6.0.html)
• switch flake8 config to use new extend-ignore option ("ignore" is an exclusive list, so overrides the default ignored list)