add lifelines optional dependency

pixi.toml

@@ -109,6 +109,7 @@ altair = "*" # used in docs/tutorials/rossman
 dask-ml = ">=2022.5.27" # used in tutorials rossman and insurance
 jupyterlab = "*"
 libpysal = "*" # used in docs/tutorials/regularization_housing_data
+lifelines = "*" # used in docs/tutorials/cox_model
 openml = "*" # used for downloading datasets in the tutorials.
 shapely = "*" # used in docs/tutorials/regularization_housing_data