Include max-line-length settings for flake8/pep8

I did this kind of on personal interest, since my emacs (using flake8)
marks every violation (meaning: line >79 chars) red, which is pretty
annoying.

I personally think 90 is a realistic guideline.

tox.ini

@@ -6,3 +6,8 @@ deps=pytest
     responses
     mock
 commands=py.test -v --pep8 mwclient tests
+
+[flake8]
+max-line-length=90
+[pep8]
+max-line-length=90