Install simplejson in devstack venv

Workaround to avoid failure due to missing osc dependency
removed in [1]

[1] https://review.opendev.org/c/openstack/python-openstackclient/+/920001

Change-Id: I3f7541e691717186b7c73f10ffabae6fc0c5c9f9

inc/python

@@ -42,6 +42,9 @@ function setup_devstack_virtualenv {
         # available on pypi.
         python$PYTHON3_VERSION -m venv --system-site-packages $DEVSTACK_VENV
         pip_install -U pip setuptools
+        #NOTE(rpittau): workaround for simplejson removal in osc
+        #  https://review.opendev.org/c/openstack/python-openstackclient/+/920001
+        pip_install -U simplejson
     fi
     if [[ ":$PATH:" != *":$DEVSTACK_VENV/bin:"* ]] ; then
         export PATH="$DEVSTACK_VENV/bin:$PATH"