Support free-threaded python 3.13+

Python 3.13 adds the ability to have a free-threaded python interpreter - which doesn't have a GIL.

py-spy doesn't currently work against free-threaded python interpreters, because:

• PyObject layout is different : instead of just containing a ob_refcnt and ob_type members - with Py_GIL_DISABLED interpreters the PyObject has additional members (threadid/mutex/ etc)
• MANAGED_DICT_OFFSET is different