restrict enforced temp coercion in normal assignments to Python objects

--HG--
extra : transplant_source : 81Y%F6%E5%B8%BF%F4%D9%97%85%93%16%9B%A5E%B2%FD%AE%5C

Cython/Compiler/Nodes.py

@@ -4433,7 +4433,7 @@ class SingleAssignmentNode(AssignmentNode):
         if use_temp or self.rhs.is_attribute:
             # (cdef) attribute access is not safe as it traverses pointers
             self.rhs = self.rhs.coerce_to_temp(env)
-        else:
+        elif self.rhs.type.is_pyobject:
             self.rhs = self.rhs.coerce_to_simple(env)
 
     def generate_rhs_evaluation_code(self, code):