Add black to pre-commit configuration

[black] is a code formatter for Python that ships with sensible defaults
and requires no configuration.

[black]: https://black.readthedocs.io/en/stable/

.pre-commit-config.yaml

@@ -22,3 +22,10 @@ repos:
     hooks:
       - id: isort
         additional_dependencies: ["toml"]
+
+  # Python code formatting
+  - repo: https://github.com/psf/black
+    rev: stable
+    hooks:
+      - id: black
+        language_version: python3.7