fixed GitHub yaml: missing cloudpickle

.github/workflows/main.yml

@@ -19,7 +19,7 @@ jobs:
       - name: Install dependencies  
         run: |
           python -m pip install --upgrade pip
-          pip install pytest pytest-benchmark enum34 numpy arrow ruamel.yaml
+          pip install pytest pytest-benchmark enum34 numpy arrow ruamel.yaml cloudpickle
       - name: Run tests
         run: |
           export PYTHONPATH=$PYTHONPATH:`pwd`