Pin python-libjuju<3.0.0

The OpenStack Charms gate has proven to be unstable when running
juju-3.x, this patch pins libjuju again to 2.9.x until the issues get
addressed.

Depends-On: openstack-charmers/zaza#652

constraints/constraints-2024.1.txt

@@ -6,7 +6,7 @@
 #   version.
 # * zaza-openstack-tests
 #
-juju>=3.1.0,<3.2.0
+juju>=2.9.0,<3.0.0
 
 # netaddr>=1.0.0 has incompatible changes, see
 # https://netaddr.readthedocs.io/en/latest/changes.html#release-1-0-0