TST: install mpi4py

Author: neutrinoceros

.github/workflows/build-test.yaml

@@ -71,7 +71,7 @@ jobs:
 
     - name: Build
       run: |
-        conda install rockstar-galaxies yt h5py
+        conda install rockstar-galaxies yt h5py mpi4py
         conda install --file=test_requirements.txt --yes
         python -m pip install -e . --no-deps