Implement optional arguments for BOOST_PYTHON_MODULE_INIT and allow the
boost::python::mod_gil_not_used() option. This sets the
Py_MOD_GIL_NOT_USED flag for the extension module.
To define a module that supports free-threaded Python, define it
like this:
BOOST_PYTHON_MODULE(my_module, boost::python::mod_gil_not_used())
{
...
}
