update CI workflow to include development requirements

Author: yucongalicechen

.github/workflows/testing.yml

@@ -43,7 +43,7 @@ jobs:
           conda config --add channels conda-forge
           conda activate test
           conda install --file requirements/requirements.txt
-          conda install pytest coverage codecov
+          conda install --file requirements/requirements-dev.txt
           pip install .
 
       - name: Validate diffpy.labpdfproc