[2.7] Doc: Add an optional obsolete header. (GH-19229)

Doc/README.txt

@@ -104,6 +104,13 @@ Then, from the ``Doc`` directory, run ::
 where ``<builder>`` is one of html, text, latex, or htmlhelp (for explanations
 see the make targets above).
 
+Deprecation header
+==================
+
+Following the sunsetting of Python 2.7, a red banner displays at the
+top of each page redirecting to the corresponding page on
+``https://docs.python.org/3/``.
+
 
 Contributing
 ============

Doc/conf.py

@@ -80,6 +80,10 @@ htmlhelp_basename = 'python' + release.replace('.', '')
 # Split the index
 html_split_index = True
 
+html_context = {
+    'outdated': True
+}
+
 
 # Options for LaTeX output
 # ------------------------

Doc/tools/templates/layout.html

@@ -1,4 +1,14 @@
 {% extends "!layout.html" %}
+{% block header %}
+{%- if outdated %}
+<div id="outdated-warning" style="padding: .5em; text-align: center; background-color: #FFBABA; color: #6A0E0E;">
+    {% trans %}This document is for an old version of Python that is {% endtrans %}<a href="https://devguide.python.org/devcycle/#end-of-life-branches">{% trans %}no longer supported{% endtrans %}</a>.
+    {% trans %}You should upgrade, and read the {% endtrans %}
+    <a href="https://docs.python.org/{{ language + '/' if language and language != 'en' else '' }}3/{{ pagename }}{{ file_suffix }}">{% trans %} Python documentation for the current stable release{% endtrans %}</a>.
+</div>
+{%- endif %}
+{% endblock %}
+
 {% block rootrellink %}
         <li><img src="{{ pathto('_static/py.png', 1) }}" alt=""
                  style="vertical-align: middle; margin-top: -1px"/></li>