Disable cython/libidn optional dep.

Index: setup.py
--- setup.py.orig
+++ setup.py
@@ -63,7 +63,7 @@ HAS_PYTHON_HEADERS = check_include('python3', 'Python.
 HAS_STRINGPREP_HEADERS = check_include('libidn', 'stringprep.h')
 
 ext_modules = None
-if HAS_PYTHON_HEADERS and HAS_STRINGPREP_HEADERS:
+if 1 == 0:
     try:
         from Cython.Build import cythonize
     except ImportError: