<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _pypsa_bus_timeseries(network, buses, timesteps):
 """ Time series in PyPSA compatible format for bus instances Set all buses except for the slack bus to voltage of 1 pu (it is assumed this setting is entirely ignored during solving the power flow problem). This slack bus is set to an operational voltage which is typically greater than nominal voltage plus a control deviation. The control deviation is always added positively to the operational voltage. For example, the operational voltage (offset) is set to 1.025 pu plus the control deviation of 0.015 pu. This adds up to a set voltage of the slack bus of 1.04 pu. .. warning:: Voltage settings for the slack bus defined by this function assume the feedin case (reverse power flow case) as the worst-case for the power system. Thus, the set point for the slack is always greater 1. Parameters network : Network The eDisGo grid topology model overall container timesteps : array_like Timesteps is an array-like object with entries of type :pandas:`pandas.Timestamp<timestamp>` specifying which time steps to export to pypsa representation and use in power flow analysis. buses : list Buses names Returns ------- :pandas:`pandas.DataFrame<dataframe>` Time series table in PyPSA format """


    # get slack bus label
    slack_bus = '_'.join(
        ['Bus', network.mv_grid.station.__repr__(side='mv')])

    # set all buses (except slack bus) to nominal voltage
    v_set_dict = {bus: 1 for bus in buses if bus != slack_bus}

    # Set slack bus to operational voltage (includes offset and control
    # deviation
    control_deviation = network.config[
        'grid_expansion_allowed_voltage_deviations'][
        'hv_mv_trafo_control_deviation']
    if control_deviation != 0:
        control_deviation_ts = \
            network.timeseries.timesteps_load_feedin_case.case.apply(
                lambda _: control_deviation if _ == 'feedin_case'
                                            else -control_deviation)
    else:
        control_deviation_ts = 0

    slack_voltage_pu = control_deviation_ts + 1 + \
                       network.config[
                           'grid_expansion_allowed_voltage_deviations'][
                           'hv_mv_trafo_offset']

    v_set_dict.update({slack_bus: slack_voltage_pu})

    # Convert to PyPSA compatible dataframe
    v_set_df = pd.DataFrame(v_set_dict, index=timesteps)

    return v_set_df