Adding cython v0.20.2 in third-party.

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

+from cpython.ref cimport PyObject
+
+cdef extern from "Python.h":
+
+    bint PyWeakref_Check(object ob)
+    # Return true if ob is either a reference or proxy object.
+
+    bint PyWeakref_CheckRef(object ob)
+    # Return true if ob is a reference object.
+
+    bint PyWeakref_CheckProxy(ob)
+    # Return true if *ob* is a proxy object.
+
+    object PyWeakref_NewRef(object ob, object callback)
+    # Return a weak reference object for the object ob.  This will
+    # always return a new reference, but is not guaranteed to create a
+    # new object; an existing reference object may be returned.  The
+    # second parameter, callback, can be a callable object that
+    # receives notification when ob is garbage collected; it should
+    # accept a single parameter, which will be the weak reference
+    # object itself. callback may also be None or NULL.  If ob is not
+    # a weakly-referencable object, or if callback is not callable,
+    # None, or NULL, this will return NULL and raise TypeError.
+
+    object PyWeakref_NewProxy(object ob, object callback)
+    # Return a weak reference proxy object for the object ob.  This
+    # will always return a new reference, but is not guaranteed to
+    # create a new object; an existing proxy object may be returned.
+    # The second parameter, callback, can be a callable object that
+    # receives notification when ob is garbage collected; it should
+    # accept a single parameter, which will be the weak reference
+    # object itself. callback may also be None or NULL.  If ob is not
+    # a weakly-referencable object, or if callback is not callable,
+    # None, or NULL, this will return NULL and raise TypeError.
+
+    PyObject* PyWeakref_GetObject(object ref)
+    # Return the referenced object from a weak reference, ref.  If the
+    # referent is no longer live, returns None.
+
+    PyObject* PyWeakref_GET_OBJECT(object ref)
+    # Similar to PyWeakref_GetObject, but implemented as a macro that
+    # does no error checking.