Configure black for Python formatting (#97)

.pre-commit-config.yaml

@@ -31,7 +31,12 @@ repos:
         additional_dependencies: [markdown-toc]
         files: .*\.md$
         exclude: ^src/jekyll/theme/
-  # Prettier should be run late so that it can re-format any prior changes.
+  # Formatters should be run late so that they can re-format any prior changes.
+  - repo: https://github.com/psf/black
+    rev: stable
+    hooks:
+      - id: black
+        language_version: python3.6
   - repo: https://github.com/prettier/prettier
     rev: 2.0.5
     hooks:

docs/project/contribution_tools.md

@@ -14,6 +14,7 @@ contributions.
 <!-- toc -->
 
 - [pre-commit](#pre-commit)
+- [black](#black)
 - [codespell](#codespell)
 - [markdown-toc](#markdown-toc)
 - [Prettier](#prettier)
@@ -42,6 +43,14 @@ To set up pre-commit:
 When modifying or adding pre-commit hooks, please run
 `pre-commit run --all-files` to see what changes.
 
+## black
+
+> **pre-commit enabled**: If you're using pre-commit, it will run this.
+> Installing and running manually is optional, but may be helpful.
+
+We use [Black](https://github.com/psf/black) to format Python code. Although
+[Prettier](#prettier) is used for most languages, it doesn't support Python.
+
 ## codespell
 
 > **pre-commit enabled**: If you're using pre-commit, it will run this.

pyproject.toml

@@ -0,0 +1,4 @@
+[tool.black]
+line-length = 80
+target-version = ['py36', 'py37', 'py38']
+include = '\.pyi?$'