Initial commit of Black as a development requirement

CONTRIBUTING.md

@@ -20,6 +20,18 @@ python -m piptools compile --output-file=requirements.txt
 python -m pip install -e ".[dev]"
 ```
 
+### How to run a code formatter?
+
+Run [Black](https://black.readthedocs.io/):
+
+```sh
+# Dry-run, showing what should be rewritten:
+python -m black --target-version=py36 --diff .
+
+# Run code format:
+python -m black --target-version=py36 .
+```
+
 ### How to run a linter?
 
 Run [Flake8](https://flake8.pycqa.org/):

setup.cfg

@@ -10,3 +10,6 @@ extend-ignore =
     # https://github.com/PyCQA/pycodestyle/issues/373 is closed.
     # https://www.flake8rules.com/rules/E203.html
     E203,
+    # Let Black manage line length by ignoring “Line too long (E203)”
+    # https://www.flake8rules.com/rules/E501.html
+    E501,

setup.py

@@ -5,6 +5,7 @@ requirements = [
 ]
 
 dev_requirements = [
+    "black",
     "flake8",
     "pip-tools",
 ]