Revert "tox -= Python3.6"

This reverts commit 469f21a9.

Even though Debian stopped shipping Python3.6 I'm now building it myself
to have test coverage for two latest Python releases.

tox.ini

 [tox]
-envlist = {py27,py37,pypy,pypy3}-{thread,gevent}
+envlist = {py27,py36,py37,pypy,pypy3}-{thread,gevent}
 
 [testenv]
 deps    =