stop generating constraints for py3.4 and py3.5

https://governance.openstack.org/tc/goals/selected/train/python3-updates.html

Change-Id: I87806f0e17e678c048035213ceebb3e073161d55

playbooks/proposal/propose_update.sh

@@ -39,8 +39,7 @@ elif [ "$OWN_PROJECT" == "requirements-constraints" ] ; then
     function update {
         $VENV/bin/generate-constraints -b blacklist.txt -p /usr/bin/python2.7 \
             -p /usr/bin/python3.6 -r global-requirements.txt \
-            --version-map 3.6:3.4 --version-map 3.6:3.5 --version-map 3.6:3.7 \
-            > $1/upper-constraints.txt
+            --version-map 3.6:3.7 > $1/upper-constraints.txt
     }
 elif [ "$OWN_PROJECT" == "devstack-plugins-list" ] ; then
     INITIAL_COMMIT_MSG="Updated from generate-devstack-plugins-list"