Apply code styles via pre-commit

- Add black and isort for Python styling
- Add doc8 for reStructuredText styling
- Add prettier for css/html/js/md/yaml styling
- Run all styling tools from pre-commit hooks
- Adjust flake8 config for compatibility with black
  (and current Django style)
- Add some other helpful pre-commit hooks
- Update editorconfig to match Django
  (with a few necessary adjustments for Anymail)
- Update `tox -e lint` to run all pre-commit hooks
- Update contributing docs

CHANGELOG.rst

@@ -25,6 +25,17 @@ Release history
 ^^^^^^^^^^^^^^^
     ..  This extra heading level keeps the ToC from becoming unmanageably long
 
+vNext
+-----
+
+*Unreleased changes*
+
+Other
+~~~~~
+* Use black, isort and doc8 to format code,
+  enforced via pre-commit. (Thanks to `@tim-schilling`_.)
+
+
 v9.0
 ----