Add flake8 check to etstool and CI

[mdickinson]

This PR:

• adds a flake8 command to etstool.py
• uses that flake8 command in Travis CI and Appveyor
• adds sufficient exclusions and ignorances to the setup.cfg to make the flake8 check pass;

The intent is to make the actual flake8 fixes (and reduce the size of the list of ignored warnings and errors) in separate PRs; this PR is focused simply on the infrastructure.

WIP because this may involve fiddling until the Appveyor and Travis runs are doing the right thing.

[codecov-io]

Codecov Report

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The diff coverage is n/a.

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  Coverage          ?   53.1%           
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  Branches          ?     518           
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  Hits              ?    2350           
  Misses            ?    2032           
  Partials          ?      43

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[mdickinson]

Working now; ready for review.

[rahulporuri]

LGTM