add black testenvs, CI stage, and contributing requirement

.travis.yml

@@ -36,5 +36,7 @@ matrix:
       env: TOXENV=flake8-tests
     - python: 3.6
       env: TOXENV=pylint-tests
+    - python: 3.6
+      env: TOXENV=black-check
 install: pip install tox
 script: tox

CONTRIBUTING.md

@@ -26,6 +26,7 @@ Contributions via pull requests are much appreciated. Before sending us a pull r
 1. You are working against the latest source on the *master* branch.
 2. You check existing open, and recently merged, pull requests to make sure someone else hasn't addressed the problem already.
 3. You open an issue to discuss any significant work - we would hate for your time to be wasted.
+4. If you modified any source code, run `tox -e blacken` and commit any changes black makes.
 
 To send us a pull request, please:
 

tox.ini

@@ -93,17 +93,24 @@ commands =
         --ignore F811,D103 \
         test/
 
-[testenv:black-check]
-basepython = {[testenv:flake8]basepython}
+[testenv:blacken]
+basepython = python3
 deps =
     black
 commands =
-    black --line-length 120 --diff \
+    black --line-length 120 \
         src/base64io/ \
         setup.py \
         doc/conf.py \
         test/
 
+[testenv:black-check]
+basepython = {[testenv:blacken]basepython}
+deps =
+    {[testenv:blacken]deps}
+commands =
+    {[testenv:blacken]commands} --diff
+
 [testenv:pylint]
 basepython = {[testenv:default-python]basepython}
 deps =