[LINTER] Enforce flake8 W605 warning

Summary:
This enforce the future-proof usage of r"string" strings.
https://lintlyci.github.io/Flake8Rules/rules/W605.html

Depends on D3461

Test Plan:

arc lint --everything

Should exhibit no failure nor warning/exception:

PYTHONWARNINGS=default::DeprecationWarning ./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3462