二级下拉表单级联
城市的二级级联，省份是一级，地级市是二级，一级省份在forms.py中写死的。
二级地级市使用ajax动态获取(暂时写死在html文件中)
页面效果如下:
但是当表单提交二级地级市总是报错:Not a valid choice 而且在表单验证之前可以打印form.data可以获取选中的值。
问题发现: 1.表单提交后，二级下拉框变为初始状态
2.在表单验证函数中输出
结果为:
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