feat(python): return Datetime/Duration with appropriate timeunit when inferring from pytype

[alexander-beedie]

Minor improvement for inference from python datetime/timedelta - include timeunit:

from polars.datatypes import py_type_to_dtype
from datetime import datetime, timedelta

py_type_to_dtype(datetime)  # >> pl.Datetime("us")
py_type_to_dtype(timedelta) # >> pl.Duration("us")

[alexander-beedie]

@ritchie46: seems to be a runner failure at the moment (also hitting #5126 and #5128) 🤔

Error: Can't use 'tar -xzf' extract archive file: 
  /home/runner/work/_actions/_temp_f291320b-9e23-4877-b85f-b1323f0b7a16/ad1d0053-5f26-412a-9e20-ab25370529a0.tar.gz. 
return code: 2.

[ritchie46]

Rerunning them now.