Docs: update tooling and outdated/broken content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/sending/anymail_additions.rst

@@ -76,8 +76,6 @@ an :ref:`unsupported feature <unsupported-features>` error.
 
     .. attribute:: envelope_sender
 
-        .. versionadded:: 2.0
-
         Set this to a `str` email address that should be used as the message's
         envelope sender. If supported by your ESP, this will become the Return-Path
         in the recipient's mailbox.

docs/sending/django_email.rst

@@ -145,10 +145,6 @@ has special handling for certain headers. Anymail replicates its behavior for co
   :class:`to <django.core.mail.EmailMessage>` list. Spoofing the :mailheader:`To` header like this
   is popular with spammers, and almost none of Anymail's supported ESPs allow it.
 
-.. versionchanged:: 2.0
-
-    Improved header-handling compatibility with Django's SMTP EmailBackend.
-
 
 .. _unsupported-features:
 

docs/sending/exceptions.rst

@@ -43,8 +43,6 @@ Exceptions
 
 .. exception:: AnymailInvalidAddress
 
-    .. versionadded:: 0.7
-
     The send call will raise a :exc:`!AnymailInvalidAddress` error if you
     attempt to send a message with invalidly-formatted email addresses in
     the :attr:`from_email` or recipient lists.