[smart_holder] Make smart holder type caster of unique_ptr accept automatic_reference

[wangxf123456]

Description

When calling a Python callback function that consumes std::unique_ptr in C++, we see RuntimeError: Invalid return_value_policy for unique_ptr. This is because when calling Python functions manually from C++ , we use return_value_policy::automatic_reference for function arguments, and the type caster of std::unique_ptr rejects this return value policy.

Pybind11 always override return_value_policy::automatic and return_value_policy::automatic_reference with return_value_policy::move for std::unique_ptr:

pybind11/include/pybind11/cast.h

Line 1138 in b644446

if (policy == return_value_policy::automatic) {

, so return_value_policy::automatic and return_value_policy::automatic_reference does not make any difference for std::unique_ptr. We should make the type caster accept both.

[rwgk]

Ignoring a test_iostream flake and Clang Dev failures fixed by PR #4767 (currently only on master).