Add black to pre-commit and requirements.

Follow on PR to reformat existing files

.pre-commit-config.yaml

@@ -11,6 +11,14 @@ repos:
     entry: mypy
     language: system
     types: [python]
+  - id: black
+    name: black
+    description: "Black: The uncompromising Python code formatter"
+    entry: black --line-length=100
+    language: python
+    require_serial: true
+    types: [python]
+    additional_dependencies: [black==23.1.0]
   - id: pytest
     name: pytest
     entry: coverage run -m pytest --typeguard-packages=strictly_typed_pandas,tests