Fix the unreleased version number is shown (refs: #3678, #3027)

doc/_templates/indexsidebar.html

@@ -3,7 +3,7 @@
   {%trans%}project{%endtrans%}</p>
 
 <h3>Download</h3>
-{% if version.endswith('a0') %}
+{% if version.endswith('+') %}
 <p>{%trans%}This documentation is for version <b><a href="changes.html">{{ version }}</a></b>, which is
   not released yet.{%endtrans%}</p>
 <p>{%trans%}You can use it from the

doc/conf.py

@@ -16,7 +16,7 @@ exclude_patterns = ['_build']
 
 project = 'Sphinx'
 copyright = '2007-2017, Georg Brandl and the Sphinx team'
-version = sphinx.__released__
+version = sphinx.__display_version__
 release = version
 show_authors = True