Update python3statement URL (#2433)

Author: hugovk

templates/python/documentation.html

@@ -81,7 +81,7 @@ <h2 class="widget-title"><span aria-hidden="true" class="icon-versions"></span>P
                         <ul>
                           <li><a href="https://www.python.org/doc/sunset-python-2/">FAQ: Sunsetting Python 2</a></li>
                           <li><a href="https://www.python.org/dev/peps/pep-0373/">Final Python 2.7 Release Schedule</a></li>
-                          <li><a href="https://python3statement.org/">Python 3 Statement</a></li>
+                          <li><a href="https://python3statement.github.io/">Python 3 Statement</a></li>
                           <li>
                             <a href="https://docs.python.org/3/howto/pyporting.html">Porting Python 2 Code to Python 3</a>
                             <ul>