Use yoga upper constraints in dev scripts

This is required to be able to install tenks. Otherwise, we try to
install Jinja2 3.1.2 which requires Python 3.7 or newer.

Change-Id: Ie497b191b6de8bc818dc4a2a12f7129a02d0fd00

dev/functions

@@ -72,7 +72,8 @@ function config_defaults {
     if [ -f "${PARENT}/../.gitreview" ]; then
       BRANCH=$(awk -F'=' '/defaultbranch/ {print $2}' "${PARENT}/../.gitreview")
       if [[ "${BRANCH}" == "" ]]; then
-        SERIES="master"
+       # TODO(priteau): Revert to master once we use Python 3.8+
+        SERIES="yoga"
       else
         SERIES="$(echo ${BRANCH} | sed 's|stable/||')"
       fi