Update tox.ini
Update tox.ini

Drop completly from tox.ini file Python3.5 for testing according to [CHANGELOG](https://github.com/un33k/python-slugify/blob/master/CHANGELOG.md#500).
More information about it at [commit which should drop Python 3.5](https://github.com/un33k/python-slugify/commit/319559cb43f239ac0d3bc7b975a78059bf4a1086)
un33k/python-slugify,un33k/python-slugify