bump required Cython version

Kwant no longer builds with Cython 0.22, and our oldest
supported operating systems ship with at least 0.24.3
as of now.

Closes #204

setup.py

@@ -167,7 +167,7 @@ def init_cython():
     global cythonize, cython_help
 
     cython_option = '--cython'
-    required_cython_version = (0, 22)
+    required_cython_version = (0, 24)
     try:
         sys.argv.remove(cython_option)
         cythonize = True