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)

hatch_build.py

@@ -0,0 +1,43 @@
+# Hatch custom build hook that generates dynamic readme.
+
+import re
+from pathlib import Path
+
+from hatchling.metadata.plugin.interface import MetadataHookInterface
+
+
+def freeze_readme_versions(text: str, version: str) -> str:
+    """
+    Rewrite links in readme text to refer to specific version.
+    (This assumes version X.Y will be tagged "vX.Y" in git.)
+    """
+    release_tag = f"v{version}"
+    return re.sub(
+        # (?<=...) is "positive lookbehind": must be there, but won't get replaced
+        # GitHub Actions build status: branch=main --> branch=vX.Y.Z:
+        r"(?<=branch[=:])main"
+        # ReadTheDocs links: /stable --> /vX.Y.Z:
+        r"|(?<=/)stable"
+        # ReadTheDocs badge: version=stable --> version=vX.Y.Z:
+        r"|(?<=version=)stable",
+        release_tag,
+        text,
+    )
+
+
+class CustomMetadataHook(MetadataHookInterface):
+    def update(self, metadata):
+        """
+        Update the project table's metadata.
+        """
+        readme_path = Path(self.root) / self.config["readme"]
+        content_type = self.config.get("content-type", "text/x-rst")
+        version = metadata["version"]
+
+        readme_text = readme_path.read_text()
+        readme_text = freeze_readme_versions(readme_text, version)
+
+        metadata["readme"] = {
+            "content-type": content_type,
+            "text": readme_text,
+        }