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)

anymail/webhooks/postal.py

@@ -31,7 +31,9 @@ except ImportError:
     # This module gets imported by anymail.urls, so don't complain about cryptography
     # missing unless one of the Postal webhook views is actually used and needs it
     error = _LazyError(
-        AnymailImproperlyInstalled(missing_package="cryptography", backend="postal")
+        AnymailImproperlyInstalled(
+            missing_package="cryptography", install_extra="postal"
+        )
     )
     serialization = error
     hashes = error