fix up demo code

Demos/libraries/setup.py

 import os
+import sys
 
 from distutils.core import setup
 from distutils.extension import Extension
-from Cython.Distutils import build_ext
+from Cython.Build import cythonize
 
 
 # For demo purposes, we build our own tiny library.
@@ -12,20 +13,19 @@ try:
     assert os.system("ar rcs libmymath.a mymath.o") == 0
 except:
     if not os.path.exists("libmymath.a"):
-        print "Error building external library, please create libmymath.a manually."
+        print("Error building external library, please create libmymath.a manually.")
         sys.exit(1)
 
 # Here is how to use the library built above. 
-ext_modules=[ 
+ext_modules = cythonize([
     Extension("call_mymath",
-              sources = ["call_mymath.pyx"],
-              include_dirs = [os.getcwd()],  # path to .h file(s)
-              library_dirs = [os.getcwd()],  # path to .a or .so file(s)
-              libraries = ['mymath'])
-]
+              sources=["call_mymath.pyx"],
+              include_dirs=[os.getcwd()],  # path to .h file(s)
+              library_dirs=[os.getcwd()],  # path to .a or .so file(s)
+              libraries=['mymath'])
+])
 
 setup(
-  name = 'Demos',
-  cmdclass = {'build_ext': build_ext},
-  ext_modules = ext_modules,
+    name='Demos',
+    ext_modules=ext_modules,
 )