add missing dateutil dependency

[noraj]

dateutil is required here

dftimewolf/dftimewolf/lib/validators/datetime_validator.py

Line 6 in 50f869b

from dateutil import parser

but never listed as dependency in pyproject.toml

it works only because dateutil is installed as a sub-dependency of other dependencies

[noraj]

PS: it is somehow included in poetry.lock as a sub-dependency of something but will still be safer to require it directly.