In setup.py, move cython to setup_requires. (#878)

* In setup.py, move cython to setup_requires and move setuptools_scm to setup_requires.

* Add back pip install of cython when building mac wheels.

* Revert changes to setuptools_scm.

* Check that the correct number of Linux wheels are produced.

* Add back pip install cython when building linux wheels.

python/setup.py

@@ -81,7 +81,6 @@ setup(name="ray",
       # The BinaryDistribution argument triggers build_ext.
       distclass=BinaryDistribution,
       install_requires=["numpy",
-                        "cython",
                         "funcsigs",
                         "click",
                         "colorama",
@@ -91,6 +90,7 @@ setup(name="ray",
                         # The six module is required by pyarrow.
                         "six >= 1.0.0",
                         "flatbuffers"],
+      setup_requires=["cython >= 0.23"],
       entry_points={"console_scripts": ["ray=ray.scripts.scripts:main"]},
       include_package_data=True,
       zip_safe=False,