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/esps/amazon_ses.rst

@@ -34,14 +34,21 @@ Installation
 ------------
 
 You must ensure the :pypi:`boto3` package is installed to use Anymail's Amazon SES
-backend. Either include the ``amazon_ses`` option when you install Anymail:
+backend. Either include the ``amazon-ses`` option when you install Anymail:
 
     .. code-block:: console
 
-        $ pip install "django-anymail[amazon_ses]"
+        $ pip install "django-anymail[amazon-ses]"
 
 or separately run ``pip install boto3``.
 
+.. versionchanged:: 10.0
+
+    In earlier releases, the "extra name" could use an underscore
+    (``django-anymail[amazon_ses]``). That now causes pip to warn
+    that "django-anymail does not provide the extra 'amazon_ses'",
+    and may result in a broken installation that is missing boto3.
+
 To send mail with Anymail's Amazon SES backend, set:
 
   .. code-block:: python