move NULL out of the way in .py compilation

Cython/Compiler/Parsing.py

@@ -615,7 +615,7 @@ def p_atom(s):
             return ExprNodes.BoolNode(pos, value=True)
         elif name == "False":
             return ExprNodes.BoolNode(pos, value=False)
-        elif name == "NULL":
+        elif name == "NULL" and not s.in_python_file:
             return ExprNodes.NullNode(pos)
         else:
             return p_name(s, name)

tests/run/pure_py.py

 import cython
 
+NULL = 5
+_NULL = NULL
+
 def test_sizeof():
     """
     >>> test_sizeof()