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/exceptions.py

@@ -171,11 +171,13 @@ class AnymailConfigurationError(ImproperlyConfigured):
 class AnymailImproperlyInstalled(AnymailConfigurationError, ImportError):
     """Exception for Anymail missing package dependencies"""
 
-    def __init__(self, missing_package, backend="<backend>"):
+    def __init__(self, missing_package, install_extra="<esp>"):
+        # install_extra must be the package "optional extras name" for the ESP
+        # (not the backend's esp_name)
         message = (
             "The %s package is required to use this ESP, but isn't installed.\n"
-            "(Be sure to use `pip install django-anymail[%s]` "
-            "with your desired ESPs.)" % (missing_package, backend)
+            '(Be sure to use `pip install "django-anymail[%s]"` '
+            "with your desired ESP name(s).)" % (missing_package, install_extra)
         )
         super().__init__(message)