tox.ini: Ignore W504 errors
tox.ini: Ignore W504 errors

Some W504 errors have been detected by flake8 after its new release.
W504 (line break after binary operator) is mostly an stylistic issue 
which we have not been very rigorous about; ignore it.

Change-Id: I203077c45b6fba00502e3e8c536da5a049493acd

wikimedia/pywikibot-core,PersianWikipedia/pywikibot-core,wikimedia/pywikibot-core