Switch to pyproject.toml packaging, using hatchling.
- Replace all uses of setup.py with updated equivalent
- BREAKING: Change extra name `amazon_ses` to
`amazon-ses`, to comply with Python packaging
name normalization
- Use hatch custom build hook to freeze version number
in readme (previously custom setup.py code)
- Move separate requirements for dev, docs, tests
into their own requirements.txt files
- Fix AnymailImproperlyInstalled to correctly refer
to package extra name
- Update testing documentation
- Update docs readme rendering to match PyPI
(and avoid setup.py)
- In tox tests, use isolated builds and update pip
- Remove AUTHORS.txt (it just referred to GitHub)
