remove mpi4py dep in serial mode.

Author: cindytsai

.github/workflows/example-test-run.yml

@@ -218,7 +218,7 @@ jobs:
           cache: 'pip'
       - run: |
           python -m pip install --upgrade pip
-          pip install numpy mpi4py yt yt-libyt pandas
+          pip install numpy yt yt-libyt pandas
 
       - name: Install other dependencies
         run: |