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/template.txt

@@ -1,11 +1,13 @@
 %(head_prefix)s
 <!--
-  This approximates PyPI.org project page styling as of 8/2020,
+  This approximates PyPI.org project page styling as of 5/2023,
   and loads their compiled CSS that was in use at that time.
 
   (Styling seems to change more often than basic page structure,
   so to update, it may be sufficient to copy in the current
-  <link rel="stylesheet" ...> tags from any live package page.)
+  <link rel="stylesheet" ...> tags from any live package page.
+  Be sure to convert or escape any percent chars in copied urls,
+  to avoid "not enough arguments for format string" errors.)
 
   This extends the docutils base template found at
   ${SITE_PACKAGES}/docutils/writers/html5_polyglot/template.txt
@@ -15,13 +17,13 @@
 %(head)s
 <!-- template (stylesheet) omitted -->
 
-<link rel="stylesheet" href="/static/css/warehouse-ltr.f2d4f304.css">
-<link rel="stylesheet" href="/static/css/fontawesome.6002a161.css">
-<link rel="stylesheet" href="/static/css/regular.98fbf39a.css">
-<link rel="stylesheet" href="/static/css/solid.c3b5f0b5.css">
-<link rel="stylesheet" href="/static/css/brands.2c303be1.css">
+<link rel="stylesheet" href="/static/css/warehouse-ltr.a42ccb04.css">
+<link rel="stylesheet" href="/static/css/fontawesome.d37999f3.css">
 <link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Source+Sans+Pro:400,400italic,600,600italic,700,700italic|Source+Code+Pro:500">
-<link rel="icon" href="/static/images/favicon.6a76275d.ico" type="image/x-icon">
+<noscript>
+  <link rel="stylesheet" href="/static/css/noscript.0673c9ea.css">
+</noscript>
+<link rel="icon" href="/static/images/favicon.35549fe8.ico" type="image/x-icon">
 
 %(body_prefix)s