make mkl-service an optional dependency

mkl-service is only a dependency for the sake of the scipy interface, which is optional

Author: ndgrigorian

.github/workflows/build_pip.yaml

@@ -53,5 +53,6 @@ jobs:
           echo "CONDA_PREFFIX is '${CONDA_PREFIX}'"
           export MKLROOT=${CONDA_PREFIX}
           pip install -e ".[test]" --no-build-isolation --verbose
+          pip install scipy pytest mkl-service
           pip list
           python -m pytest -v mkl_fft/tests