feat(python): add map_dict method for Series

[alexander-beedie]

Expose the useful new map_dict functionality on Series (via @expr_dispatch):

import polars as pl

country_lookup = {
    "JPN": "Japan",
    "FRA": "France",
    "NLD": "Netherlands",
}
s = pl.Series("iso3166", ["FRA", "???", "JPN", "NLD"])

s.map_dict(
    country_lookup, 
    default = s,
).rename("country_name")

# shape: (4,)
# Series: 'country_name' [str]
# [
#     "France"
#     "???"
#     "Japan"
#     "Netherlands"
# ]

[ritchie46]

Thanks! Docs entry?😊

[alexander-beedie]

Thanks! Docs entry?😊

Lol... my kryptonite; had realised and was adding it as you typed 🤣