test/upcast.cpp does not compile since python 3.10

Start by reading test/upcast.cpp at lines 16-17 and reproducing the reported compilation with Python 3.10 or 3.13 headers. Compare the Py_REFCNT usage with the changed Python C API, then verify that test/upcast.cpp compiles successfully with the newer Python versions.

revision: 3e7be69e1e405e1d5ddd232c69c024ee441592c5

Since Python 3.10, compilation of test/upcast.cpp fails.

https://github.com/boostorg/python/blob/3e7be69e1e405e1d5ddd232c69c024ee441592c5/test/upcast.cpp#L16-L17

$ g++ -Iinclude -I/opt/miniconda3/envs/py310/include/python3.10 test/upcast.cpp
In file included from /usr/include/boost/detail/lightweight_test.hpp:15,
                 from ./test/upcast.cpp:7:
./test/upcast.cpp: In function 'int main()':
/opt/miniconda3/envs/py310/include/python3.10/object.h:133:33: error: lvalue required as unary '&' operand
  133 | #define Py_REFCNT(ob) _Py_REFCNT(_PyObject_CAST_CONST(ob))
      |                       ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
./test/upcast.cpp:16:17: note: in expansion of macro 'Py_REFCNT'
   16 |     BOOST_TEST(&Py_REFCNT(boost::python::upcast<PyObject>(&o)) == &Py_REFCNT(&o));
      |                 ^~~~~~~~~
/opt/miniconda3/envs/py310/include/python3.10/object.h:133:33: error: lvalue required as unary '&' operand
  133 | #define Py_REFCNT(ob) _Py_REFCNT(_PyObject_CAST_CONST(ob))
      |                       ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
./test/upcast.cpp:16:68: note: in expansion of macro 'Py_REFCNT'
   16 |     BOOST_TEST(&Py_REFCNT(boost::python::upcast<PyObject>(&o)) == &Py_REFCNT(&o));
      |                                                                    ^~~~~~~~~
/opt/miniconda3/envs/py310/include/python3.10/object.h:133:33: error: lvalue required as unary '&' operand
  133 | #define Py_REFCNT(ob) _Py_REFCNT(_PyObject_CAST_CONST(ob))
      |                       ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
./test/upcast.cpp:17:17: note: in expansion of macro 'Py_REFCNT'
   17 |     BOOST_TEST(&Py_REFCNT(boost::python::upcast<PyObject>(&y)) == &Py_REFCNT(&y));
      |                 ^~~~~~~~~
/opt/miniconda3/envs/py310/include/python3.10/object.h:133:33: error: lvalue required as unary '&' operand
  133 | #define Py_REFCNT(ob) _Py_REFCNT(_PyObject_CAST_CONST(ob))
      |                       ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
./test/upcast.cpp:17:68: note: in expansion of macro 'Py_REFCNT'
   17 |     BOOST_TEST(&Py_REFCNT(boost::python::upcast<PyObject>(&y)) == &Py_REFCNT(&y));
      |                                                                    ^~~~~~~~~

$ g++ -Iinclude -I/usr/include/python3.13 test/upcast.cpp
In file included from /usr/include/boost/detail/lightweight_test.hpp:15,
                 from test/upcast.cpp:7:
test/upcast.cpp: In function 'int main()':
/usr/include/python3.13/object.h:330:34: error: lvalue required as unary '&' operand
  330 | #  define Py_REFCNT(ob) Py_REFCNT(_PyObject_CAST(ob))
      |                         ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
test/upcast.cpp:16:17: note: in expansion of macro 'Py_REFCNT'
   16 |     BOOST_TEST(&Py_REFCNT(boost::python::upcast<PyObject>(&o)) == &Py_REFCNT(&o));
      |                 ^~~~~~~~~
/usr/include/python3.13/object.h:330:34: error: lvalue required as unary '&' operand
  330 | #  define Py_REFCNT(ob) Py_REFCNT(_PyObject_CAST(ob))
      |                         ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
test/upcast.cpp:16:68: note: in expansion of macro 'Py_REFCNT'
   16 |     BOOST_TEST(&Py_REFCNT(boost::python::upcast<PyObject>(&o)) == &Py_REFCNT(&o));
      |                                                                    ^~~~~~~~~
/usr/include/python3.13/object.h:330:34: error: lvalue required as unary '&' operand
  330 | #  define Py_REFCNT(ob) Py_REFCNT(_PyObject_CAST(ob))
      |                         ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
test/upcast.cpp:17:17: note: in expansion of macro 'Py_REFCNT'
   17 |     BOOST_TEST(&Py_REFCNT(boost::python::upcast<PyObject>(&y)) == &Py_REFCNT(&y));
      |                 ^~~~~~~~~
/usr/include/python3.13/object.h:330:34: error: lvalue required as unary '&' operand
  330 | #  define Py_REFCNT(ob) Py_REFCNT(_PyObject_CAST(ob))
      |                         ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~
test/upcast.cpp:17:68: note: in expansion of macro 'Py_REFCNT'
   17 |     BOOST_TEST(&Py_REFCNT(boost::python::upcast<PyObject>(&y)) == &Py_REFCNT(&y));
      |                                                                    ^~~~~~~~~

This is because Py_REFCNT() is changed to the inline static function.

Unfortunately, the CI images use python upto 3.8, so the issue has not been found.