Bumping minimum tox version

and remove outdated testenv (py36-local)

Change-Id: Id17c5a8370933804637e160c93bf15a0c63658ae
Signed-off-by: Ivan Anfimov <lazekteam@gmail.com>

tox.ini

@@ -1,6 +1,6 @@
 [tox]
-envlist = pep8,py3,py36-local,eslint,karma-local,docs-local,releasenotes
-minversion = 2.3.2
+envlist = pep8,py3,eslint,karma-local,docs-local,releasenotes
+minversion = 3.18.0
 
 [testenv]
 usedevelop = True
@@ -39,23 +39,11 @@ max-complexity = 20
 [testenv:cover]
 commands = python setup.py test --coverage --testr-args='{posargs}'
 
-# NOTE(shu-mutow): On CI infra, horizon will be installed
-# according to job setting. but on local, we need to install
-# horizon from master branch.
-[testenv:py36-local]
-basepython = python3.6
-commands =
-  {[testenv:hz-local]commands}
-  {[testenv]commands}
-
 [testenv:eslint]
 commands =
   npm install
   npm run lint
 
-# NOTE(shu-mutow): The "postinstall" script on package.json will install horizon
-# from master branch into python3.x environment for testing javascripts.
-# Horizon from master is needed to be cloned into ../horizon on both local and CI.
 [testenv:karma]
 commands =
   npm install