Add support for Python 3.7 and fix Flake8

[hugovk]

Add support for Python 3.7, it needs Xenial on Travis CI.

(Also sudo: is no longer required: https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration)

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A new release of Flake8 has caused the QA build job to fail with:

W504 line break after binary operator

Fixing these cause:

W503 line break before binary operator

W503 and W504 are not compatible with each other, so one needs ignoring. W503 is not compatible with PEP 8, so ignore it. (If you prefer it the other way, let's ignore the other one.)

[coveralls]

Coverage remained the same at 71.156% when pulling 3d491e8 on hugovk:update-ci into a4a9670 on sybrenstuvel:master.

[sybrenstuvel]

Thanks! I've cherrypicked two of the four commits from this PR, namely the Flake8 ones. I'm migrating the package management to Poetry and will add Python 3.7 support there too.