[Travis] flake8 was updated to 3.6.0

[guedou]

flake8 tests are broken thanks to 'W504 line break after binary operator' and ' W605 invalid escape sequence'.

See https://travis-ci.org/secdev/scapy/jobs/445176377

[guedou]

@gpotter2 could you have a look at W605 ? It looks trivial to fix but only impact Windows. I don't want to silently breal something.

[gpotter2]

Done, see #1666

[gpotter2]

FTR see #1660