Adding cython v0.20.2 in third-party.

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

Index: third_party/cython/src/Cython/Includes/cpython/ref.pxd
diff --git a/third_party/cython/src/Cython/Includes/cpython/ref.pxd b/third_party/cython/src/Cython/Includes/cpython/ref.pxd
new file mode 100644
index 0000000000000000000000000000000000000000..f41c9a5dcd0838d7fa8c82f0e52123d7209e78c7
--- /dev/null
+++ b/third_party/cython/src/Cython/Includes/cpython/ref.pxd
@@ -0,0 +1,56 @@
+cdef extern from "Python.h":
+    ctypedef struct PyTypeObject
+    ctypedef struct PyObject:
+        Py_ssize_t ob_refcnt
+        PyTypeObject *ob_type
+    cdef PyTypeObject *Py_TYPE(object)
+
+
+    #####################################################################
+    # 3. Reference Counts
+    #####################################################################
+    # The macros in this section are used for managing reference counts of Python objects.
+    void Py_INCREF(object o)
+    # Increment the reference count for object o. The object must not
+    # be NULL; if you aren't sure that it isn't NULL, use
+    # Py_XINCREF().
+
+    void Py_XINCREF(PyObject* o)
+    # Increment the reference count for object o. The object may be NULL, in which case the macro has no effect.
+
+    void Py_DECREF(object o)
+    # Decrement the reference count for object o. The object must not
+    # be NULL; if you aren't sure that it isn't NULL, use
+    # Py_XDECREF(). If the reference count reaches zero, the object's
+    # type's deallocation function (which must not be NULL) is
+    # invoked.
+
+    # Warning: The deallocation function can cause arbitrary Python
+    # code to be invoked (e.g. when a class instance with a __del__()
+    # method is deallocated). While exceptions in such code are not
+    # propagated, the executed code has free access to all Python
+    # global variables. This means that any object that is reachable
+    # from a global variable should be in a consistent state before
+    # Py_DECREF() is invoked. For example, code to delete an object
+    # from a list should copy a reference to the deleted object in a
+    # temporary variable, update the list data structure, and then
+    # call Py_DECREF() for the temporary variable.
+
+    void Py_XDECREF(PyObject* o)
+    # Decrement the reference count for object o. The object may be
+    # NULL, in which case the macro has no effect; otherwise the
+    # effect is the same as for Py_DECREF(), and the same warning
+    # applies.
+
+    void Py_CLEAR(PyObject* o)
+    # Decrement the reference count for object o. The object may be
+    # NULL, in which case the macro has no effect; otherwise the
+    # effect is the same as for Py_DECREF(), except that the argument
+    # is also set to NULL. The warning for Py_DECREF() does not apply
+    # with respect to the object passed because the macro carefully
+    # uses a temporary variable and sets the argument to NULL before
+    # decrementing its reference count.
+    # It is a good idea to use this macro whenever decrementing the
+    # value of a variable that might be traversed during garbage
+    # collection.
+