ci: add pandas dependency for serializer tests

- Install pandas in CI test environment
- Fixes ModuleNotFoundError for pandas in test_serializer.py
- Both numpy and pandas are optional runtime deps but required for tests

Author: luohf-infinilabs

.github/workflows/ci.yml

@@ -60,7 +60,7 @@ jobs:
         run: |
           python -m pip install --upgrade pip
           python -m pip install -e .
-          python -m pip install pytest pytest-cov mock numpy
+          python -m pip install pytest pytest-cov mock numpy pandas
       - name: Run Unit Tests
         run: |
           # Run basic import test