Check style in CI.

.github/workflows/ubuntu.yml

@@ -34,10 +34,10 @@ jobs:
       run: |
         python -m pip install --upgrade pip tox
 
-    #- name: Check style
-    #  if: matrix.python-version == '3.6'
-    #  run: |
-    #    tox -e style
+    - name: Check style
+      if: matrix.python-version == '3.6'
+      run: |
+        tox -e style
 
     - name: Run tests
       run: |

pyproject.toml

@@ -0,0 +1,20 @@
+# NOTE: you have to use single-quoted strings in TOML for regular expressions.
+# It's the equivalent of r-strings in Python.  Multiline strings are treated as
+# verbose regular expressions by Black.  Use [ ] to denote a significant space
+# character.
+
+[tool.black]
+line-length = 88
+target-version = ['py35', 'py36', 'py37', 'py38']
+include = '\.pyi?$'
+exclude = '''
+/(
+    \.eggs
+  | \.git
+  | \.tox
+  | \.venv
+  | _build
+  | build
+  | dist
+)/
+'''

tox.ini

@@ -1,6 +1,6 @@
 ; Run all tests with "tox".
 [tox]
-envlist = py35, py36, py37, py38
+envlist = py35, py36, py37, py38, style
 basepython = python3
 skip_missing_interpreters = true
 
@@ -9,3 +9,17 @@ deps =
   pytest
 commands =
   pytest src/tests
+
+[testenv:style]
+skipsdist = true
+deps =
+  black
+commands =
+  black --check .
+
+[testenv:fix-style]
+skipsdist = true
+deps =
+  black
+commands =
+  black .