Upgrade PyScss to version 1.2.1 and pin it <1.3 for Horizon

PyScss fixed an important bug in parsing Bootstrap 3.2.0
(Kronuz/pyScss#287). While investigating
this book with the upstream developers, we also became aware that
version 1.3 will break backwards compatibility with Django-PyScss
that we are using, so I'm pinning the version.

Change-Id: Ic4c23dcbf35dd55916401bba2b1bc3bf4baa4522
Partial-bug: #1370137

global-requirements.txt

@@ -40,7 +40,7 @@ jsonschema>=2.0.0,<3.0.0
 kazoo>=1.3.1
 keystonemiddleware>=1.0.0
 lesscpy>=0.9j
-pyscss>=1.2.0  # MIT License
+pyscss>=1.2.1,<1.3  # MIT License
 django-pyscss>=1.0.2  # BSD License (2 clause)
 kombu>=2.4.8
 libvirt-python>=1.2.5                   # LGPLv2+