tox.ini: Avoid pep8-naming 0.6
tox.ini: Avoid pep8-naming 0.6

It's causing failures in pywikibot-core-tox-docker test.
Upstream issue: https://github.com/PyCQA/pep8-naming/issues/53

Bug: T193922
Change-Id: Ifed2becc1b73b4e061fb5a762b36f651a3cbb4b7

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