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/__init__.py

@@ -1,7 +1,17 @@
-# Expose package version at root of package
-from django import VERSION as DJANGO_VERSION
+from ._version import VERSION, __version__
 
-from ._version import VERSION, __version__  # NOQA: F401
+__all__ = [
+    "VERSION",
+    "__version__",
+]
 
-if DJANGO_VERSION < (3, 2, 0):
-    default_app_config = "anymail.apps.AnymailBaseConfig"
+try:
+    import django
+except ImportError:
+    # (don't require django just to get package version)
+    pass
+else:
+    if django.VERSION < (3, 2, 0):
+        # (No longer required -- and causes deprecation warning -- in Django 3.2+)
+        default_app_config = "anymail.apps.AnymailBaseConfig"
+        __all__.append("default_app_config")