PEP 227 implementation

A cell contains a reference to a single PyObject.  It could be
implemented as a mutable, one-element sequence, but the separate type
has less overhead.
diff --git a/Include/Python.h b/Include/Python.h
index 3086552..b771c8c 100644
--- a/Include/Python.h
+++ b/Include/Python.h
@@ -81,6 +81,7 @@
 #include "cobject.h"
 #include "traceback.h"
 #include "sliceobject.h"
+#include "cellobject.h"
 
 #include "codecs.h"
 #include "pyerrors.h"