Fix pyScss version in lower-constraints.txt
Fix pyScss version in lower-constraints.txt

Horizon requires  pyScss>=1.3.7 [1] but lower-constraints.txt
in senlin-dashboard uses pyScss 1.3.4 which can cause the
failure in future. So this patch update pyScss version in
lower-constraints.txt to sync the change.

[1] https://review.opendev.org/#/c/715241/

Change-Id: If3bef074ca9be89b80b81185397ccb35511f102b

openstack/senlin-dashboard,stackforge/senlin-dashboard,stackforge/senlin-dashboard,openstack/senlin-dashboard,openstack/senlin-dashboard,stackforge/senlin-dashboard,openstack/senlin-dashboard