Operators couldn't return boost::python::object

When I trying to define operator which returns boost::python::object I get error ` ‘struct boost::python::converter::arg_to_python’ has no member named ‘release’`

`def` method for operators doesn't accept additional arguments to define other converters.

首先复现一个返回 boost::python::object 的运算符，并跟踪提及 arg_to_python 和 release 的转换器错误。调查 def 方法如何处理运算符转换器；当此类运算符能够工作并接受所需的转换器参数时即可完成。

由索引模型根据 Issue 内容生成。