try including cython in ci build

.github/workflows/ci-tests.yml

@@ -26,8 +26,8 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install flake8 pytest numpy
-        pip install numpy orbitize
+        pip install flake8 pytest numpy cython
+        pip install orbitize
         pip install -r requirements.txt
         python setup.py develop
         git lfs pull