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/conf.py

@@ -13,6 +13,8 @@ import os
 import sys
 from pathlib import Path
 
+from anymail import VERSION as PACKAGE_VERSION
+
 ON_READTHEDOCS = os.environ.get("READTHEDOCS", None) == "True"
 DOCS_PATH = Path(__file__).parent
 PROJECT_ROOT_PATH = DOCS_PATH.parent
@@ -22,15 +24,6 @@ PROJECT_ROOT_PATH = DOCS_PATH.parent
 # documentation root, use os.path.abspath to make it absolute, like shown here.
 sys.path.insert(0, PROJECT_ROOT_PATH.resolve())
 
-# define __version__ and __minor_version__ from ../anymail/_version.py,
-# but without importing from anymail (which would make docs dependent on Django, etc.)
-__version__ = "UNSET"
-__minor_version__ = "UNSET"
-version_path = PROJECT_ROOT_PATH / "anymail/_version.py"
-code = compile(version_path.read_text(), version_path, "exec")
-exec(code)
-
-
 # -- General configuration -----------------------------------------------------
 
 # If your documentation needs a minimal Sphinx version, state it here.
@@ -61,10 +54,10 @@ copyright = "Anymail contributors"
 # |version| and |release|, also used in various other places throughout the
 # built documents.
 #
-# The short X.Y version.
-version = __minor_version__
 # The full version, including alpha/beta/rc tags.
-release = __version__
+release = ".".join(PACKAGE_VERSION)
+# The short X.Y version.
+version = ".".join(PACKAGE_VERSION[:2])
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.