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)

CHANGELOG.rst

@@ -33,9 +33,25 @@ vNext
 Breaking changes
 ~~~~~~~~~~~~~~~~
 
+* **Amazon SES:** The "extra name" for installation must now be spelled with
+  a hyphen rather than an underscore: ``django-anymail[amazon-ses]``.
+  Be sure to update any dependencies specification (pip install, requirements.txt,
+  etc.) that had been using ``[amazon_ses]``.
+
 * Require Python 3.7 or later.
+
 * Require urllib3 1.25 or later (released 2019-04-29).
 
+Other
+~~~~~
+
+* Modernize packaging. (Change from setup.py and setuptools
+  to pyproject.toml and hatchling.) Other than the ``amazon-ses``
+  naming normalization noted above, the new packaging should have
+  no impact. If you have trouble installing django-anymail v10 where
+  v9 worked, please report an issue including the exact install
+  command and pip version you are using.
+
 
 v9.2
 -----