Added isort to tox envlist

Author: Anton Ruhlov

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py35,py36,py37,pypy,pypy3,docs,readme,bandit
+envlist = py35,py36,py37,pypy,pypy3,docs,readme,bandit,isort
 
 [testenv]
 deps=
@@ -34,3 +34,10 @@ skipsdist=True
 deps=bandit
 commands=bandit -b bandit.json -r oauthlib/
 whitelist_externals=bandit
+
+[testenv:isort]
+basepython = python3.7
+usedevelop = false
+deps = isort
+changedir = {toxinidir}
+commands = isort --recursive --check-only --diff oauthlib tests