Add libsass

The django-pyscss and pyscss libraries currently being used in Horizon
are not currently maintained and have an increasing number of problems
that we have to work around. We want to instead switch to using the
libsass library, which is using a C library of the same name under the
hood, which in turn contains the official implementation of the sass and
scss parsers.

python3-libsass

 * Is the library actively maintained?
    The library is a python wrapper for the official C libsass library,
    and it's updated with every libsass release.
    https://github.com/sass/libsass-python/commits/main/
 * Is the library good code?
    As far as I can tell, it's a correctly written python wrapper for C
    library. It includes tests.
 * Is the library license compatible?
    Yes, it's MIT license.
 * Is the library already packaged in the distros we target (Ubuntu
   latest / Fedora latest)?
    Yes.
    https://packages.fedoraproject.org/pkgs/python-libsass/python3-libsass/
    https://packages.ubuntu.com/oracular/python3-libsass
 * Is the function of this library already covered by other libraries in global-requirements.txt?
    Yes, we are replacing python3-pyscss because it's unmaintained and
    has problems with recent versions of Python.
 * Is the library required for OpenStack project or related dev or infrastructure setup? (Answer to this should be Yes, of course) Which?
    It's needed for openstack-dashboard, aka Horizon.
 * If the library release is managed by the Openstack release process does it use the cycle-with-intermediary release type?
    It's not managed by the OpenStack.

We no longer need django-libsass, we replaced it with our own code.

Change-Id: I6d5c895e36766c9b1e1bb41f4c98468ba1b51a64

global-requirements.txt

@@ -36,6 +36,7 @@ django-compressor  # MIT
 django-debreach  # BSD
 django-formtools  # BSD
 django-pyscss  # BSD License (2 clause)
+libsass  # MIT
 Django<4.3  # BSD
 # eventlet is not compatibile with 2.0.0: https://github.com/eventlet/eventlet/issues/619
 dnspython!=2.0.0,!=2.2.0  # http://www.dnspython.org/LICENSE

upper-constraints.txt

@@ -288,6 +288,7 @@ importlib-metadata===6.11.0;python_version>='3.10'
 oslo.middleware===6.2.0
 XStatic-mdi===1.6.50.2
 django-pyscss===2.0.3
+libsass===0.23.0
 uritemplate===4.1.1
 docutils===0.21.2
 threadpoolctl===3.5.0