Docs about formatting the code with ``black``

See <https://pypi.org/project/black/>.

.drone.yml

@@ -22,6 +22,13 @@
       # tox only pycodestyle & py27 because python3 requires a genshi trunk from svn...
       - tox -e pycodestyle,py27
 
+  black:
+    image: houzefaabba/python3-black
+    pull: true
+    commands:
+      # Ensure our code is black-valid <https://pypi.org/project/black/>.
+      - black --check .
+
   notify:
     image: plugins/slack
     channel: dev

docs/source/hacking.rst

@@ -18,4 +18,9 @@
 Code style
 ----------
 
-TODO
+We let ``black`` <https://pypi.org/project/black/> take care of everything
+here.
+
+Follow its install guide to get it (requires Python 3.6).
+Alternatively, the <https://hub.docker.com/r/houzefaabba/python3-black/> Docker
+image contains Python 3.6 with ``black`` installed inside.

pyproject.toml

+# See <https://pypi.org/project/black/>.
+
+# black defaults to "line-length = 88"; we prefer "line-length = 79" for now to
+# please our various tools / IDEs / linters.
+[tool.black]
+line-length = 79