Wrap linting into one test env.

tox.ini

@@ -1,14 +1,12 @@
 [tox]
-skipsdist = True
 usedevelop = True
 minversion = 1.8
 envlist =
-    flake8-py{27,35},
-    readme-py{27,35},
+    lint,
     # the version matrix according to the Django docs:
     # https://docs.djangoproject.com/en/2.0/faq/install/#what-python-version-can-i-use-with-django
     docs-py35-dj{18,19,110,111,20},
-    tests-py{27,33,34,35}-dj18,
+    tests-py{27,34,35}-dj18,
     tests-py{27,34,35}-dj{19,110},
     tests-py{27,34,35,36}-dj111,
     tests-py{34,35,36}-dj20,
@@ -32,14 +30,22 @@ deps =
     dj111: -ctests/constraints/django-1.11.txt
     fl011: -ctests/constraints/flask-0.11.txt
     fl012: -ctests/constraints/flask-0.12.txt
-    readme: readme_renderer
-    flake8: flake8
 commands =
     python --version
     tests: pytest {posargs:tests}
     docs: sphinx-build -b html -d {envtmpdir}/doctrees docs {envtmpdir}/html
-    readme: python setup.py check -r -s
-    flake8: flake8 src/dockerflow
+
+[testenv:lint]
+basepython = python3.6
+deps =
+    flake8
+    twine
+    check-manifest
+commands =
+    flake8 src/dockerflow tests/
+    check-manifest -v
+    python setup.py sdist
+    twine check dist/*
 
 [flake8]
 exclude=.tox