Major overhaul:

- transition to `pyproject.toml` config
- include license (Apache-2.0)
- add dev tools
- add documentation config (`mkdocs` based)
- turn `README.md` into a symlink to docs `index.md`
- add convenience scripts for testing and linting

requirements/dev.txt

@@ -1,2 +1,8 @@
 -r common.txt
+black
+build
 coverage
+flake8
+mkdocs-material
+mkdocstrings[python]
+mypy