CI: use musicode branch of mupif instead of master

.github/workflows/tests.yml

@@ -26,7 +26,7 @@ jobs:
         sudo apt-get update && sudo apt-get -y install python3-wheel python3-pip mongodb-org
         python -m pip install -q uv
         uv pip install --system flake8
-        uv pip install --system git+https://github.com/mupif/mupif.git@master
+        uv pip install --system git+https://github.com/mupif/mupif.git@Musicode
         uv pip install --system -r requirements.txt
     - name: Lint with flake8
       run: |