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)

README.rst

@@ -5,9 +5,8 @@ Anymail: Django email integration for transactional ESPs
     * Github: project page, exactly as it appears here
     * Docs: shared-intro section gets included in docs/index.rst
             quickstart section gets included in docs/quickstart.rst
-    * PyPI: project page (via setup.py long_description),
-            with several edits to freeze it to the specific PyPI release
-            (see long_description_from_readme in setup.py)
+    * PyPI: project page (via pyproject.toml readme; see also
+            hatch_build.py which edits in the release version number)
     You can use docutils 1.0 markup, but *not* any Sphinx additions.
     GitHub rst supports code-block, but *no other* block directives.