Docs: update tooling and outdated/broken content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/_static/anymail-theme.css

@@ -1,31 +1,9 @@
 /* Anymail modifications to sphinx-rtd-theme styles */
 
-/* List item spacing: https://github.com/rtfd/sphinx_rtd_theme/issues/590 */
-/*        Regression: https://github.com/rtfd/sphinx_rtd_theme/issues/705 */
-.rst-content .section ol li p:last-child,
-.rst-content .section ul li p:last-child {
-  margin-bottom: 24px;
-}
-
-
-/* Pygments shell/console prompt: */
-.highlight .gp {
-  /* The RTD theme shows prompts the same color as other code.
-     We want to distinguish them (like most Pygments themes do). */
-  color: #d14;
-
-  /* Exclude prompt characters when copying from highlighted code. */
-  user-select: none;
-  -moz-user-select: none;
-  -webkit-user-select: none;
-  -ms-user-select: none;
-}
-
 
 /* Sticky table first column (used for ESP feature matrix) */
 table.sticky-left td:first-of-type,
 table.sticky-left th:first-of-type {
-  position: -webkit-sticky; /* Safari */
   position: sticky;
   left: 0;
   background-color: #fcfcfc; /* override transparent from .wy-table td */
@@ -36,7 +14,6 @@ table.sticky-left th:first-of-type[colspan] > p {
      since those always wrap a rubric <p> (in the specific table that uses this),
      just make the <p> sticky within the <td>. */
   display: inline-block;
-  position: -webkit-sticky; /* Safari */
   position: sticky;
   left: 17px; /* (.wy-table $table-padding-size) + (docutils border="1" in html) */
 }
@@ -57,12 +34,12 @@ table.sticky-left th:first-of-type[colspan] > p {
             https://github.com/rtfd/sphinx_rtd_theme/issues/92
 */
 .rst-content a code.xref {
-  color: #2980B9;
-  font-weight: inherit;
+  color: inherit;
+  /*font-weight: inherit;*/
 }
 .rst-content a:hover code.xref {
-  color: #3091d1;
+  color: inherit;
 }
 .rst-content a:visited code.xref {
-  color: #9B59B6;
+  color: inherit;
 }

docs/_static/version-alert.js

@@ -27,4 +27,8 @@ function warnOnLatestVersion() {
   parent.insertBefore(warning, parent.firstChild);
 }
 
-document.addEventListener('DOMContentLoaded', warnOnLatestVersion);
+if (document.readyState === 'loading') {
+  document.addEventListener('DOMContentLoaded', warnOnLatestVersion);
+} else {
+  warnOnLatestVersion();
+}