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())
    {
        ...
    }

Update scripts to use actions/setup-python to install different Python
versions.  Add run-faber.sh and get-py-env.py scripts.  Add
test-ubuntu-py-ver.yml CI script to test with different Python versions.

We don't need test-ubuntu-py-ver.yml since we can use "include".  Test
Python 3.14t as well.  More Python versions can be tested now we are
using setup-python.