Don't require Cython at setup time when generated C files are available

setup.py

@@ -1,13 +1,5 @@
 import os
 from setuptools import setup, Extension
-from Cython.Build import cythonize
-
-EXT_MODULES = [
-    Extension(
-        'numba_special.function_pointers',
-        [os.path.join('numba_special', 'function_pointers.pyx')]
-    )
-]
 
 
 def get_long_description():
@@ -20,16 +12,35 @@ def get_long_description():
     return long_description
 
 
+def get_extension_modules():
+    try:
+        from Cython.Build import cythonize
+    except ImportError:
+        return [
+            Extension(
+                'numba_special.function_pointers',
+                [os.path.join('numba_special', 'function_pointers.c')]
+            )
+        ]
+
+    return cythonize([
+        Extension(
+            'numba_special.function_pointers',
+            [os.path.join('numba_special', 'function_pointers.pyx')]
+        )
+    ])
+
+
 setup(
     name='numba_special',
     description="Numba overloads for SciPy's special functions",
     long_description=get_long_description(),
     long_description_content_type='text/markdown',
-    version='0.1.0rc2',
+    version='0.1.0rc3',
     author='Josh Wilson',
     url='https://github.com/person142/numba_special',
     packages=['numba_special'],
     package_data={'numba_special': ['signatures.json']},
-    ext_modules=cythonize(EXT_MODULES),
-    install_requires=['cython', 'numba', 'scipy']
+    ext_modules=get_extension_modules(),
+    install_requires=['numba', 'scipy']
 )