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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def _set_state(pkg, state):
    '''
    Change package state on the opkg database

    The state can be any of:

     - hold
     - noprune
     - user
     - ok
     - installed
     - unpacked

    This command is commonly used to mark a specific package to be held from
    being upgraded, that is, to be kept at a certain version.

    Returns a dict containing the package name, and the new and old
    versions.
    '''
    ret = {}
    valid_states = ('hold', 'noprune', 'user', 'ok', 'installed', 'unpacked')
    if state not in valid_states:
        raise SaltInvocationError('Invalid state: {0}'.format(state))
    oldstate = _get_state(pkg)
    cmd = ['opkg', 'flag']
    cmd.append(state)
    cmd.append(pkg)
    _out = __salt__['cmd.run'](cmd, python_shell=False)

    # Missing return value check due to opkg issue 160
    ret[pkg] = {'old': oldstate,
                'new': state}
    return ret