update generate constraints to py38,39

3.10 will come later when there is an image that supports all three

Change-Id: I663de65a971337fce65a51f0e5a7f959b8da2c03
Signed-off-by: Matthew Thode <mthode@mthode.org>

playbooks/proposal/propose_update.sh

@@ -36,9 +36,9 @@ elif [ "$OWN_PROJECT" == "requirements-constraints" ] ; then
     VENV=$(readlink -f .tox/venv)
     $VENV/bin/pip install -e .
     function update {
-        $VENV/bin/generate-constraints -b blacklist.txt -p /usr/bin/python3.6 \
-            -p /usr/bin/python3.8 -r global-requirements.txt \
-            > $1/upper-constraints.txt
+        $VENV/bin/generate-constraints -b blacklist.txt -p /usr/bin/python3.8 \
+            -p /usr/bin/python3.9 \
+            -r global-requirements.txt > $1/upper-constraints.txt
     }
 elif [ "$OWN_PROJECT" == "devstack-plugins-list" ] ; then
     INITIAL_COMMIT_MSG="Updated from generate-devstack-plugins-list"