integration: Pins juju to 3.1 for integration tests

Pins the installed version of juju to 3.1.

Additionally, switches microk8s version to 1.27-strict/stable, as
juju 3.0 and newer requires microk8s to be strictly confined.

Updated the Python juju dependency.

.github/workflows/ci.yaml

@@ -36,6 +36,8 @@ jobs:
         uses: charmed-kubernetes/actions-operator@main
         with:
           provider: microk8s
+          juju-channel: 3.1/stable
+          channel: 1.27-strict/stable
           microk8s-addons: "storage dns rbac ingress"
 
       - name: Run integration tests

tox.ini

@@ -56,7 +56,7 @@ commands =
 description = Run integration tests
 deps =
     pytest
-    juju
+    juju < 3.2.0
     pytest-operator
     tenacity
     requests