Update testing.yml

Author: tschaume

.github/workflows/testing.yml

@@ -32,7 +32,7 @@ jobs:
 
       - name: Install Python dependencies
         run: |
-          python -m pip install --upgrade pip
+          python -m pip install --upgrade "pip<25.3"
           pip install -r requirements/requirements-${{ matrix.os }}_py${{ matrix.python-version }}.txt
           pip install -r requirements/requirements-${{ matrix.os }}_py${{ matrix.python-version }}_extras.txt