Cells are not VAR objects.

Noted by Jason Orendorff, SF #520768.

Bug fix candidate for 2.1 & 2.2.

Include/cellobject.h

diff --git a/Include/cellobject.h b/Include/cellobject.h
index cc4a159..f3eef55 100644
--- a/Include/cellobject.h
+++ b/Include/cellobject.h
@@ -7,7 +7,7 @@
 #endif
 
 typedef struct {
-	PyObject_VAR_HEAD
+	PyObject_HEAD
 	PyObject *ob_ref;
 } PyCellObject;