For the free-threaded build, it is not safe use borrowed references.
Another thread could deallocate the object and cause the reference to
become invalid.  Replace API calls that borrow references with strong
reference APIs.

For the free-threaded build (and possibly the debug build), it is not
safe to call Py_DECREF() if there is no valid Python thread-state.

Add pymutex.hpp which implements a re-entrant mutex on top of Python's
PyMutex.  Add BOOST_PYTHON_LOCK_STATE() macro that uses RAII to lock
mutable global state as required.

This indicates that the free-threaded build of Python can keep the
GIL disabled when the module is loaded.  Without this module flag,
importing the module will cause the GIL to be re-enabled.  A warning
is emitted if this happens.

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.