On latest Xarray(>2024.04.0), you can do:
import xarray as xr
from xarray.groupers import SeasonResampler
ds = xr.tutorial.open_dataset("air_temperature")
ds.resample(time=SeasonResampler(["DJ"])).mean()
https://docs.xarray.dev/en/latest/user-guide/time-series.html#handling-seasons