Add a "dev version" warning banner to the latest version of documentation (GH-4583)

docs/_templates/layout.html

 {% extends "!layout.html" %}
 
+{% block header %}
+{%- if development %}
+<div id="development-warning" style="padding: .5em; text-align: center; background-color: #FFBABA; color: #6A0E0E;">
+    {% trans %}This version of the documentation is for the in-development branch of Cython.
+    For the stable version, see {% endtrans %}
+    <a href="/en/stable/{{ pagename }}{{ file_suffix }}">{% trans %}here{% endtrans %}</a>.
+</div>
+{%- endif %}
+{% endblock %}
+
 {% block relbar1 %}
 {{ super() }}
 {% if pagename != "src/donating" %}

docs/conf.py

@@ -184,6 +184,10 @@ html_context = {
     'css_files': ['_static/css/tabs.css'],
 }
 
+development = 'a' in release or 'b' in release
+
+html_context = {'development': development}
+
 # If not '', a 'Last updated on:' timestamp is inserted at every page bottom,
 # using the given strftime format.
 #html_last_updated_fmt = '%b %d, %Y'