use interned Python string constant instead of calling through __Pyx_GetAttrString()

Cython/Compiler/ExprNodes.py

@@ -9958,8 +9958,9 @@ class DocstringRefNode(ExprNode):
         pass
 
     def generate_result_code(self, code):
-        code.putln('%s = __Pyx_GetAttrString(%s, "__doc__"); %s' % (
+        code.putln('%s = __Pyx_GetAttr(%s, %s); %s' % (
             self.result(), self.body.result(),
+            code.intern_identifier(StringEncoding.EncodedString("__doc__")),
             code.error_goto_if_null(self.result(), self.pos)))
         code.put_gotref(self.result())