Require black and isort for contributions (#3329)

* Add explanation to CONTRIBUTNG.md
* Add sample pre-commit script
* Check for correctly formatted files in CI

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

.travis.yml

@@ -27,5 +27,12 @@ jobs:
     - name: "check file consistency"
       script: ./tests/check_consistent.py
     - name: "flake8"
-      install: pip install -r requirements-tests-py3.txt
+      install: pip install $(grep flake8 requirements-tests-py3.txt)
       script: flake8
+    - name: "black"
+      install: pip install $(grep black requirements-tests-py3.txt)
+      script: black --check --diff stdlib third_party
+  allow_failures:
+    - name: "isort"
+      install: pip install $(grep isort requirements-tests-py3.txt)
+      script: isort --check-only --diff --recursive stdlib third_party