CI: Use mizani main branch for testing

.github/workflows/testing.yml

@@ -39,6 +39,7 @@ jobs:
           # Install as an editable so that the coverage path
           # is predicable
           pip install -e ".[extra,test]"
+          pip install git+https://github.com/has2k1/mizani#egg=mizani
 
       - name: Environment Information
         shell: bash