Add python 3.6 in tox.ini

[farseerfc]

Python 3.6 is now released, this change added python3.6 in tox.ini

[andriisoldatenko]

@farseerfc What about update in all places?
E.g. https://github.com/getpelican/pelican/blob/master/.travis.yml

[farseerfc]

Sorry I didn't add travis config. I don't know when travis will support python 3.6

[ingwinlu]

Hi, travis already supports it in their own weird way that does not play that nice with our setup.

See #2094 for a complete update.

[farseerfc]

@ingwinlu hi, thanks for telling me!