TimeDelta misleading 'precision' argument & v4 breaking change

The docstring is correct:

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:param precision: The time unit used for (de)serialization. Must be one of 'weeks',
   'days', 'hours', 'minutes', 'seconds', 'milliseconds' or 'microseconds'.

it's the units (de)serialised to; it doesn't effect precision.

That is, 123 microseconds with the default "seconds" precision will serialise to 0.000123, not 0.

This seems to be a breaking change in v4.0.0, perhaps somehow related to the date/time/isoformat change, I think in 0c901108945573c825e33960707426c80e19290e as previously it would return a timedelta(microseconds=123) in the _serialize method, but now returns a float via utils.timedelta_to_microseconds divided by the 'precision''s mapping value, i.e. 1000000 for seconds.