Missing build-time Cython dependency.

requirements.txt

+Cython
+cytoolz

setup.py

@@ -10,6 +10,6 @@ setup(
     license="MIT",
     # packages=["libsmallrna"],
     ext_modules = cythonize("libsmallrna/libsmallrna.pyx"),
-    install_requires=["cytoolz"],
+    install_requires=["Cython", "cytoolz"],
     zip_safe=False,
     packages=find_packages())