Flake8 had a new release which raise new issues, namely:

W504: line break after binary operator
F841: local variable 'ex' is assigned to but never used
W605: invalid escape sequence
F821: undefined name 'buffer' (noqa seems the only way to avoid this false positive)

Also pin flake8>=3.6 in our tests and make explicit that we use python3 to run
flake8 tests.