Adding cython v0.20.2 in third-party.

third_party/cython/src/Cython/Includes/cpython/iterator.pxd

+cdef extern from "Python.h":
+
+    ############################################################################
+    # 6.5 Iterator Protocol
+    ############################################################################
+    bint PyIter_Check(object o)
+    # Return true if the object o supports the iterator protocol.
+
+    object PyIter_Next(object o)
+    # Return value: New reference.
+    # Return the next value from the iteration o. If the object is an
+    # iterator, this retrieves the next value from the iteration, and
+    # returns NULL with no exception set if there are no remaining
+    # items. If the object is not an iterator, TypeError is raised, or
+    # if there is an error in retrieving the item, returns NULL and
+    # passes along the exception.
+
+    # To write a loop which iterates over an iterator, the C code should look something like this:
+    # PyObject *iterator = PyObject_GetIter(obj);
+    # PyObject *item;
+    # if (iterator == NULL) {
+    # /* propagate error */
+    # }
+    # while (item = PyIter_Next(iterator)) {
+    # /* do something with item */
+    # ...
+    # /* release reference when done */
+    # Py_DECREF(item);
+    # }
+    # Py_DECREF(iterator);
+    # if (PyErr_Occurred()) {
+    # /* propagate error */
+    # }
+    # else {
+    # /* continue doing useful work */
+    # }