Force installation of pip < 26 for now

.github/workflows/modoboa.yml

@@ -203,10 +203,10 @@ jobs:
       - uses: actions/checkout@v4
         with:
           fetch-depth: 0
-      - name: Set up Python 3.12
+      - name: Set up Python 3.13
         uses: actions/setup-python@v5
         with:
-          python-version: '3.12'
+          python-version: '3.13'
       - name: Build frontend
         shell: bash -l {0}
         run: |
@@ -220,7 +220,7 @@ jobs:
       - name: Build packages
         run: |
           sudo apt-get install librrd-dev rrdtool libssl-dev
-          python -m pip install --upgrade "pip!=25.3" build pip-tools
+          python -m pip install --upgrade "pip<26" build pip-tools
           pip-compile -o requirements.txt pyproject.toml
           pip install -r requirements.txt
           cd modoboa