Revert "Run propose-update-constraints under Python3.6"

This reverts commit c1de884d33.

propose-updates-constraints != propose-updates.  The later does what I want, the former is used by the release-team after releasing a new library.

#eggonface

Change-Id: I75280b23e2e60800af45dc2343c82008ec9971a9

playbooks/proposal/propose_update.sh

@@ -38,8 +38,8 @@ elif [ "$OWN_PROJECT" == "requirements-constraints" ] ; then
     $VENV/bin/pip install -e .
     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 \
+            -p /usr/bin/python3 -r global-requirements.txt \
+            --version-map 3.4:3.5 --version-map 3.5:3.4 \
             > $1/upper-constraints.txt
     }
 elif [ "$OWN_PROJECT" == "devstack-plugins-list" ] ; then

zuul.d/jobs.yaml

@@ -945,7 +945,6 @@
 - job:
     name: propose-update-constraints
     parent: propose-updates
-    nodeset: ubuntu-bionic
     description: |
       Update constraint files for requirements project with a
       proposed change.