change to python 3.6+ supported from 3.4+

Author: rouilj

doc/installation.txt

@@ -172,7 +172,7 @@ can remove the tracker using ``rm -f`` on the ``demo`` directory.
 Prerequisites
 =============
 
-Roundup requires Python 2.7 or 3.4 or newer with a functioning anydbm
+Roundup requires Python 2.7 or 3.6 or newer with a functioning anydbm
 or sqlite module. The version installed by most vendors should work if
 it meets the version requirements. If necessary, you can download the
 latest version from https://www.python.org/. It is highly recommended