Method subinterpreter

include/pybind11/subinterpreter.h:50–50

empty/unusable, but move-assignable. use create() to create a subinterpreter.

public:
    /// empty/unusable, but move-assignable.  use create() to create a subinterpreter.
    subinterpreter() = default;

    subinterpreter(subinterpreter const &copy) = delete;
    subinterpreter &operator=(subinterpreter const &copy) = delete;