Doc: 3.8 is now stable. (GH-16790) (GH-16794)

(cherry picked from commit 4504b450)
Co-authored-by: Julien Palard <julien@palard.fr>

Doc/tools/templates/indexsidebar.html

@@ -3,7 +3,7 @@
 <h3>{% trans %}Docs by version{% endtrans %}</h3>
 <ul>
   <li><a href="https://docs.python.org/3.9/">{% trans %}Python 3.9 (in development){% endtrans %}</a></li>
-  <li><a href="https://docs.python.org/3.8/">{% trans %}Python 3.8 (pre-release){% endtrans %}</a></li>
+  <li><a href="https://docs.python.org/3.8/">{% trans %}Python 3.8 (stable){% endtrans %}</a></li>
   <li><a href="https://docs.python.org/3.7/">{% trans %}Python 3.7 (stable){% endtrans %}</a></li>
   <li><a href="https://docs.python.org/3.6/">{% trans %}Python 3.6 (security-fixes){% endtrans %}</a></li>
   <li><a href="https://docs.python.org/3.5/">{% trans %}Python 3.5 (security-fixes){% endtrans %}</a></li>