Return installed backends.

    Backends are installed python packages named pyvisa-<something> where <something>
    is the name of the backend.

    :rtype: list

def list_backends():
    """Return installed backends.

    Backends are installed python packages named pyvisa-<something> where <something>
    is the name of the backend.

    :rtype: list
    """
    return ['ni'] + [name for (loader, name, ispkg) in pkgutil.iter_modules()
                     if name.startswith('pyvisa-') and not name.endswith('-script')]