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/_version.py

@@ -1,7 +1,7 @@
-VERSION = (9, 2)
+# Don't import this file directly (unless you are a build system).
+# Instead, load version info from the package root.
 
 #: major.minor.patch or major.minor.devN
-__version__ = ".".join([str(x) for x in VERSION])
+__version__ = "10.0.dev0"
 
-#: Sphinx's X.Y "version"
-__minor_version__ = ".".join([str(x) for x in VERSION[:2]])
+VERSION = __version__.split(",")