Set up pre-commit to run black

Call it automatically via tox

.pre-commit-config.yaml

@@ -0,0 +1,5 @@
+repos:
+  - repo: https://github.com/python/black
+    rev: 19.3b0
+    hooks:
+      - id: black

tox.ini

@@ -37,6 +37,11 @@ commands =
     # pytest-cov doesn't seem to play nice with -p
     coverage run -p -m pytest tests examples
 
+[testenv:stylecheck]
+deps = pre-commit
+skip_install = true
+commands = pre-commit run --all-files --show-diff-on-failure
+
 [testenv:docs-html]
 deps =
     -r docs/requirements.txt