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)

tox.ini

@@ -28,9 +28,15 @@ envlist =
     djangoDev-py{310,311}-all
     # ... then partial installation (limit extras):
     django42-py311-{none,amazon_ses,postal}
+# tox requires isolated builds to use pyproject.toml build config:
+isolated_build = True
 
 [testenv]
+args_are_paths = false
+# Download latest version of pip/setuptools available on each Python version:
+download = true
 deps =
+    -rtests/requirements.txt
     django30: django~=3.0.0
     django31: django~=3.1.0
     django32: django~=3.2.0
@@ -40,10 +46,10 @@ deps =
     django50: django~=5.0.0a0
     djangoDev: https://github.com/django/django/tarball/main
 extras =
-    # install [test] extras, unconditionally
-    test
-    # install [esp_name] extras only when testing "all" or esp_name factor
-    all,amazon_ses: amazon_ses
+    # Install [esp-name] extras only when testing "all" or esp_name factor.
+    # (Only ESPs with extra dependencies need to be listed here.
+    # Careful: tox factors (on the left) use underscore; extra names use hyphen.)
+    all,amazon_ses: amazon-ses
     all,postal: postal
 setenv =
     # tell runtests.py to limit some test tags based on extras factor
@@ -61,11 +67,9 @@ setenv =
 ignore_outcome =
     # CI that wants to handle errors itself can set TOX_OVERRIDE_IGNORE_OUTCOME=false
     djangoDev: {env:TOX_OVERRIDE_IGNORE_OUTCOME:true}
-args_are_paths = false
-# Upgrade pip/wheel/setuptools:
-download = true
 commands_pre =
     python -VV
+    python -m pip --version
     python -c 'import django; print("Django", django.__version__)'
 commands =
     python runtests.py {posargs}
@@ -98,25 +102,22 @@ commands =
     pre-commit run --all-files
 
 [testenv:docs]
-basepython = python3.8
-skip_install = true
+basepython = python3.10
 passenv =
     CONTINUOUS_INTEGRATION
 	# (but not any of the live test API keys)
 setenv =
     DOCS_BUILD_DIR={envdir}/_html
-whitelist_externals = /bin/bash
 deps =
     -rdocs/requirements.txt
 commands_pre =
     python -VV
     sphinx-build --version
 commands =
-    # Verify README.rst as used in setup.py long_description:
-    python setup.py check --restructuredtext --strict
     # Build and verify docs:
     sphinx-build -W -b dirhtml docs {env:DOCS_BUILD_DIR}
-    # Build README.rst into html:
-    /bin/bash -c 'python setup.py --long-description \
-                  | rst2html5.py --config=docs/_readme/docutils.cfg \
-                  > {env:DOCS_BUILD_DIR}/readme.html'
+    # Build and verify package metadata readme.
+    # Errors here are in README.rst:
+    python docs/_readme/render.py \
+        --package django-anymail \
+        --out {env:DOCS_BUILD_DIR}/readme.html