Missing Cython to build apistar service

[tompaton]

I got an error running docker-compose build:

    File "<string>", line 1, in <module>
    File "/tmp/pip-install-t5i6nxmf/scikit-learn/setup.py", line 290, in <module>
      setup_package()
    File "/tmp/pip-install-t5i6nxmf/scikit-learn/setup.py", line 286, in setup_package
      setup(**metadata)
    File "/usr/local/lib/python3.6/site-packages/numpy/distutils/core.py", line 135, in setup
      config = configuration()
    File "/tmp/pip-install-t5i6nxmf/scikit-learn/setup.py", line 174, in configuration
      config.add_subpackage('sklearn')
    File "/usr/local/lib/python3.6/site-packages/numpy/distutils/misc_util.py", line 1024, in add_subpackage
      caller_level = 2)
    File "/usr/local/lib/python3.6/site-packages/numpy/distutils/misc_util.py", line 993, in get_subpackage
      caller_level = caller_level + 1)
    File "/usr/local/lib/python3.6/site-packages/numpy/distutils/misc_util.py", line 930, in _get_configuration_from_setup_py
      config = setup_module.configuration(*args)
    File "sklearn/setup.py", line 66, in configuration
      config.add_subpackage('utils')
    File "/usr/local/lib/python3.6/site-packages/numpy/distutils/misc_util.py", line 1024, in add_subpackage
      caller_level = 2)
    File "/usr/local/lib/python3.6/site-packages/numpy/distutils/misc_util.py", line 993, in get_subpackage
      caller_level = caller_level + 1)
    File "/usr/local/lib/python3.6/site-packages/numpy/distutils/misc_util.py", line 930, in _get_configuration_from_setup_py
      config = setup_module.configuration(*args)
    File "sklearn/utils/setup.py", line 8, in configuration
      from Cython import Tempita
  ModuleNotFoundError: No module named 'Cython'
  ----------------------------------------
  ERROR: Failed building wheel for scikit-learn
  Running setup.py clean for scikit-learn
Failed to build scikit-learn

I added cython to the second-last pip install in services/apistar/Dockerfile and it built successfully.