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)

tests/test_sendinblue_integration.py

@@ -120,10 +120,10 @@ class SendinBlueBackendIntegrationTests(AnymailTestMixin, SimpleTestCase):
             "attachment": [
                 {
                     "name": "attachment1.txt",
-                    # URL where Sendinblue can download
-                    # the attachment content while sending:
-                    "url": "https://raw.githubusercontent.com/anymail"
-                    "/django-anymail/main/AUTHORS.txt",
+                    # URL where Sendinblue can download the attachment content while
+                    # sending (must be content-type: text/plain):
+                    "url": "https://raw.githubusercontent.com/anymail/django-anymail/"
+                    "main/docs/_readme/template.txt",
                 }
             ]
         }