don't pick up cython if present at build time; there are pregenerated files
anyway, and 1) py-gevent wants 3.0a versions of cython, and 2) hash filenames
are created which depend on a digest of compiler directives, which differs
to the pregenerated ones and it ends up with both getting installed

Index: _setuputils.py
--- _setuputils.py.orig
+++ _setuputils.py
@@ -216,11 +216,8 @@ def _dummy_cythonize(extensions, **_kwargs):
         extension.sources[:] = sources
     return extensions
 
-try:
-    from Cython.Build import cythonize
-except ImportError:
-    # The .c files had better already exist.
-    cythonize = _dummy_cythonize
+# The .c files had better already exist.
+cythonize = _dummy_cythonize
 
 def cythonize1(ext):
     # All the directories we have .pxd files