CI: Add condition to decide cache type

Author: ByteOtter

.github/workflows/pytests.yml

@@ -17,10 +17,24 @@ jobs:
         with:
           fetch-depth: 2
 
+      - name: Activate caching based on lockfile
+        id: lockfile
+        run: |
+          if [ -f poetry.lock ]; then
+            echo "cache_type=poetry" >> $GITHUB_OUTPUT
+          elif [ -f requirements.txt ]; then
+            echo "cache_type=pip" >> $GITHUB_OUTPUT
+          else
+            # no lockfile present: create empty requirements.txt and use pip caching
+            touch requirements.txt
+            echo "cache_type=pip" >> $GITHUB_OUTPUT
+          fi
+
       - name: Set up Python
         uses: actions/setup-python@v5
         with:
           python-version: '3.13'
+          cache: ${{ steps.lockfile.outputs.cache_type }}
 
       - name: Install Poetry
         uses: snok/install-poetry@v1