Break circular import in _data_pipeline.py

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

src/frequenz/sdk/microgrid/_data_pipeline.py

@@ -22,7 +22,6 @@
 from .._internal._channels import ChannelRegistry
 from ..actor._actor import Actor
 from ..timeseries import ResamplerConfig
-from ..timeseries._grid_frequency import GridFrequency
 from ..timeseries._voltage_streamer import VoltageStreamer
 from ..timeseries.grid import Grid
 from ..timeseries.grid import get as get_grid
@@ -35,6 +34,7 @@
 #
 # pylint: disable=import-outside-toplevel
 if typing.TYPE_CHECKING:
+    from ..timeseries._grid_frequency import GridFrequency
     from ..timeseries.battery_pool import BatteryPool
     from ..timeseries.battery_pool._battery_pool_reference_store import (
         BatteryPoolReferenceStore,
@@ -140,6 +140,8 @@ def __init__(
 
     def frequency(self) -> GridFrequency:
         """Return the grid frequency measuring point."""
+        from ..timeseries._grid_frequency import GridFrequency
+
         if self._frequency_instance is None:
             self._frequency_instance = GridFrequency(
                 self._data_sourcing_request_sender(),