test/upcast.cpp does not compile since python 3.10

revision: 3e7be69e1e405e1d5ddd232c69c024ee441592c5

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

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

```console
$ 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(&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(&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(&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(&y)) == &Py_REFCNT(&y));
| ^~~~~~~~~
```

```console
$ 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(&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(&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(&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(&y)) == &Py_REFCNT(&y));
| ^~~~~~~~~
```

This is because [`Py_REFCNT()` is changed to the inline static function](https://docs.python.org/3.10/c-api/structures.html#c.Py_REFCNT).

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