support StringNode in compile time expressions

Cython/Compiler/ExprNodes.py

@@ -881,6 +881,9 @@ class StringNode(PyConstNode):
     def calculate_result_code(self):
         return self.result_code
         
+    def compile_time_value(self, env):
+        return self.value
+
 
 class LongNode(AtomicExprNode):
     #  Python long integer literal