Add black in the tox matrix.

dev-requirements.txt

@@ -3,3 +3,4 @@ tox
 pytest
 flake8
 Flask-Testing
+black

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py37,py36,py35,py34,docs,lint
+envlist = py37,py36,py35,docs,flake8,black
 skip_missing_interpreters = True
 
 [testenv]
@@ -21,7 +21,13 @@ deps =
     -rdocs/requirements.txt
 changedir = {toxinidir}
 
-[testenv:lint]
+[testenv:black]
+commands = black --check --target-version=py34 .
+deps =
+    -rdev-requirements.txt
+changedir = {toxinidir}
+
+[testenv:flake8]
 commands = flake8 ihatemoney
 deps =
     -rdev-requirements.txt
@@ -30,9 +36,12 @@ changedir = {toxinidir}
 [flake8]
 exclude = migrations
 max_line_length = 100
+extend-ignore =
+    # See https://github.com/PyCQA/pycodestyle/issues/373
+    E203,
 
 [travis]
 python =
   3.5: py35
-  3.6: py36, docs, lint
+  3.6: py36, docs, black, flake8
   3.7: py37