Change python version to 3.5

On Ubuntu Xenial there is no way to install python3.4
Use value specified in PYTHON3_VERSION

Change-Id: Ibc69b1c8270bdd240c82cf2acfdfd0730ef0f182
diff --git a/inc/python b/inc/python
index 9de2831..e4cfab8 100644
--- a/inc/python
+++ b/inc/python
@@ -370,7 +370,7 @@
 # Install python3 packages
 function install_python3 {
     if is_ubuntu; then
-        apt_get install python3.4 python3.4-dev
+        apt_get install python${PYTHON3_VERSION} python${PYTHON3_VERSION}-dev
     fi
 }