fix bytes.join() in Py<=2.5

Cython/Utility/StringTools.c

@@ -661,7 +661,11 @@ static CYTHON_INLINE char __Pyx_PyBytes_GetItemInt(PyObject* bytes, Py_ssize_t i
 #endif
 
 #if CYTHON_COMPILING_IN_CPYTHON
-#define __Pyx_PyBytes_Join _PyBytes_Join
+    #if PY_MAJOR_VERSION < 3
+    #define __Pyx_PyBytes_Join _PyString_Join
+    #else
+    #define __Pyx_PyBytes_Join _PyBytes_Join
+    #endif
 #else
 static CYTHON_INLINE PyObject* __Pyx_PyBytes_Join(PyObject* sep, PyObject* values); /*proto*/
 #endif