tox cleanup (#2877)

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py{33,34,35,36,37}-{cdh,hdp,core,contrib,apache,aws,gcloud,postgres,unixsocket,azureblob,dropbox}, visualiser, pypy-scheduler, docs, flake8
+envlist = py{33,34,35,36,37}-{cdh,hdp,core,contrib,apache,aws,gcloud,postgres,unixsocket,azureblob,dropbox}, visualiser, docs, flake8
 skipsdist = True
 
 [testenv]
@@ -23,7 +23,7 @@ deps =
     postgres: psycopg2<3.0
     mysql-connector-python>=8.0.12
     gcloud: google-api-python-client>=1.4.0,<2.0
-    py33,py34,py35,py36,py37: avro-python3
+    avro-python3
     gcloud: google-auth==1.4.1
     gcloud: google-auth-httplib2==0.0.3
     google-compute-engine
@@ -101,21 +101,20 @@ max-line-length = 160
 builtins = unicode
 
 [testenv:flake8]
+basepython=python3
 deps =
     flake8>=3.2.0
 commands =
     flake8 --exclude=doc,.tox
     flake8 --max-line-length=100 --ignore=E265 doc
 
-[testenv:autopep8]
-deps = autopep8
-commands = autopep8 --ignore E309,E501 -a -i -r luigi test examples bin
-
 [testenv:isort]
+basepython=python3
 deps = isort
 commands = isort -w 120 -rc luigi test examples bin
 
 [testenv:docs]
+basepython=python3
 # Build documentation using sphinx.
 # Call this using `tox -e docs`.
 deps =