use cythonize() instead of build_ext override in tutorial examples

docs/examples/tutorial/fib1/setup.py

 from distutils.core import setup
-from distutils.extension import Extension
-from Cython.Distutils import build_ext
+from Cython.Build import cythonize
 
 setup(
-    cmdclass = {'build_ext': build_ext},
-    ext_modules = [Extension("fib", ["fib.pyx"])]
+    ext_modules=cythonize("fib.pyx"),
 )
-

docs/examples/tutorial/primes/setup.py

 from distutils.core import setup
-from distutils.extension import Extension
-from Cython.Distutils import build_ext
+from Cython.Build import cythonize
 
 setup(
-    cmdclass = {'build_ext': build_ext},
-    ext_modules = [Extension("primes", ["primes.pyx"])]
+    ext_modules=cythonize("primes.pyx"),
 )
-