add version map for python3.6 to 3.7

This prevents installing unconstrainted versions of libraries for
python3.7.  When it was unmapped there was no constraint for it for
certian libraries (some libraries had diferent versions for each version
of python).

Change-Id: I0a5ecc715142756a9ec37fca559fad691651430c

playbooks/proposal/propose_update.sh

@@ -39,7 +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 3.6:3.4 --version-map 3.6:3.5 --version-map 3.6:3.7 \
             > $1/upper-constraints.txt
     }
 elif [ "$OWN_PROJECT" == "devstack-plugins-list" ] ; then