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/_readme/render.py

@@ -0,0 +1,109 @@
+#!/usr/bin/env python
+# Render a README file (roughly) as it would appear on PyPI
+
+import argparse
+import sys
+from importlib.metadata import PackageNotFoundError, metadata
+from pathlib import Path
+from typing import Dict, Optional
+
+import readme_renderer.rst
+from docutils.core import publish_string
+from docutils.utils import SystemMessage
+
+# Docutils template.txt in our directory:
+DEFAULT_TEMPLATE_FILE = Path(__file__).with_name("template.txt").absolute()
+
+
+def get_package_readme(package: str) -> str:
+    # Note: "description" was added to metadata in Python 3.10
+    return metadata(package)["description"]
+
+
+class ReadMeHTMLWriter(readme_renderer.rst.Writer):
+    translator_class = readme_renderer.rst.ReadMeHTMLTranslator
+
+    def interpolation_dict(self) -> Dict[str, str]:
+        result = super().interpolation_dict()
+        # clean the same parts as readme_renderer.rst.render:
+        clean = readme_renderer.rst.clean
+        result["docinfo"] = clean(result["docinfo"])
+        result["body"] = result["fragment"] = clean(result["fragment"])
+        return result
+
+
+def render(source_text: str, warning_stream=sys.stderr) -> Optional[str]:
+    # Adapted from readme_renderer.rst.render
+    settings = readme_renderer.rst.SETTINGS.copy()
+    settings.update(
+        {
+            "warning_stream": warning_stream,
+            "template": DEFAULT_TEMPLATE_FILE,
+            # Input and output are text str (we handle decoding/encoding):
+            "input_encoding": "unicode",
+            "output_encoding": "unicode",
+            # Exit with error on docutils warning or above.
+            # (There's discussion of having readme_renderer ignore warnings;
+            # this ensures they'll be treated as errors here.)
+            "halt_level": 2,  # (docutils.utils.Reporter.WARNING_LEVEL)
+            # Report all docutils warnings or above.
+            # (The readme_renderer default suppresses this output.)
+            "report_level": 2,  # (docutils.utils.Reporter.WARNING_LEVEL)
+        }
+    )
+
+    writer = ReadMeHTMLWriter()
+
+    try:
+        return publish_string(
+            source_text,
+            writer=writer,
+            settings_overrides=settings,
+        )
+    except SystemMessage:
+        warning_stream.write("Error rendering readme source.\n")
+        return None
+
+
+def main(argv=None):
+    parser = argparse.ArgumentParser(
+        description="Render readme file as it would appear on PyPI"
+    )
+    input_group = parser.add_mutually_exclusive_group(required=True)
+    input_group.add_argument(
+        "-p", "--package", help="Source readme from package's metadata"
+    )
+    input_group.add_argument(
+        "-i",
+        "--input",
+        help="Source readme.rst file ('-' for stdin)",
+        type=argparse.FileType("r"),
+    )
+    parser.add_argument(
+        "-o",
+        "--output",
+        help="Output file (default: stdout)",
+        type=argparse.FileType("w"),
+        default="-",
+    )
+
+    args = parser.parse_args(argv)
+    if args.package:
+        try:
+            source_text = get_package_readme(args.package)
+        except PackageNotFoundError:
+            print(f"Package not installed: {args.package!r}", file=sys.stderr)
+            sys.exit(2)
+        if source_text is None:
+            print(f"No metadata readme for {args.package!r}", file=sys.stderr)
+            sys.exit(2)
+    else:
+        source_text = args.input.read()
+    rendered = render(source_text)
+    if rendered is None:
+        sys.exit(2)
+    args.output.write(rendered)
+
+
+if __name__ == "__main__":
+    main()