Docs: update tooling and outdated/broken content

* Update docs build config
  * Specify Sphinx and theme versions in docs/requirements.txt
    (tox had been using latest; RTD had pinned older versions)
  * Update docs builds for Python 3 and recent Sphinx
  * Remove obsolete sphinx-rtd-theme patches
  * Add .readthedocs.yml
  * Move some JS to async (now that Sphinx supports that)
  * Fix version-alert.js for async loading
* Remove docs versionadded/changed info older than v3.0
* Fix broken GitHub issue links in changelog (never worked?)
* Add setup long_description_content_type
* Drop poorly maintained AUTHORS.txt 
  (just point to GitHub contributors page)

docs/_readme/template.txt

@@ -1,20 +1,25 @@
 %(head_prefix)s
 <!--
-  This approximates PyPI.org project page styling as of 8/2019
-  (and relies on their compiled CSS that was available at that time).
+  This approximates PyPI.org project page styling as of 8/2020,
+  and loads their compiled CSS that was in use at that time.
 
-  Base template is ${SITE_PACKAGES}/docutils/writers/html5_polyglot/template.txt
+  (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.)
+
+  This extends the docutils base template found at
+  ${SITE_PACKAGES}/docutils/writers/html5_polyglot/template.txt
 -->
 
 <base href="https://pypi.org/">
 %(head)s
 <!-- template (stylesheet) omitted -->
 
-<link rel="stylesheet" href="/static/css/warehouse.a1809af1.css">
-<link rel="stylesheet" href="/static/css/fontawesome.4b73fd92.css">
-<link rel="stylesheet" href="/static/css/regular.19624371.css">
-<link rel="stylesheet" href="/static/css/solid.f478cfb1.css">
-<link rel="stylesheet" href="/static/css/brands.1ea560bf.css">
+<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="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">
 
@@ -22,16 +27,16 @@
 
 <main id="#content">
 
-  <section class="banner">
+  <div class="banner">
     <div class="package-header">
     <div class="package-header__left">
       %(body_pre_docinfo)s
       %(docinfo)s
     </div>
     </div>
-  </section>
+  </div>
 
-  <section>
+  <div>
     <div class="tabs-container">
     <div class="vertical-tabs">
     <div class="vertical-tabs__panel">
@@ -45,7 +50,7 @@
     </div>
     </div>
     </div>
-  </section>
+  </div>
 
 </main>
 %(body_suffix)s