Modernize packaging

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)

docs/requirements.txt

@@ -1,4 +1,7 @@
+# Packages required only for building docs
+
 # (Pygments defaulted "python" to Python 2 before v2.5.0; it doesn't use semver)
 Pygments~=2.9.0
+readme-renderer~=37.3
 sphinx~=4.0
 sphinx-rtd-theme~=0.5.2