Travis run is failing with flake8 errors

[edmorley]

Travis had been switched off for some years, and after re-enabling it just now, the runs are failing with:

$ flake8
./bob/cli.py:73:1: W293 blank line contains whitespace
./bob/cli.py:78:1: E302 expected 2 blank lines, found 1
./bob/models.py:32:1: E302 expected 2 blank lines, found 1
./bob/models.py:135:31: E711 comparison to None should be 'if cond is not None:'
./bob/models.py:138:79: E261 at least two spaces before inline comment
./bob/models.py:145:31: E261 at least two spaces before inline comment
./bob/models.py:153:33: E261 at least two spaces before inline comment
./bob/models.py:173:31: E711 comparison to None should be 'if cond is not None:'
./bob/utils.py:16:1: E302 expected 2 blank lines, found 1
./bob/utils.py:60:1: E302 expected 2 blank lines, found 1
./bob/utils.py:62:1: W293 blank line contains whitespace
./bob/utils.py:63:21: E261 at least two spaces before inline comment
./bob/utils.py:65:1: W293 blank line contains whitespace
./bob/utils.py:66:39: E261 at least two spaces before inline comment
./bob/utils.py:67:67: E261 at least two spaces before inline comment
./bob/utils.py:68:1: W293 blank line contains whitespace
./bob/utils.py:70:1: W293 blank line contains whitespace
./bob/utils.py:71:37: E261 at least two spaces before inline comment
./bob/utils.py:73:1: E302 expected 2 blank lines, found 1

eg:
https://travis-ci.com/github/heroku-python/bob-builder/jobs/393824894#L530