Three years ago Paul Ganssle wrote a comparison about time zone handling between pytz and python-dateutil. In this article he shows how it's easy to use pytz in an incorrect way that is hard to spot because it's almost correct:
import pytz
from datetime import datetime, timedelta
NYC = pytz.timezone('America/New_York')
dt = datetime(2018, 2, 14, 12, tzinfo=NYC)
print(dt)
# 2018-02-14 12:00:00-04:56
(link to part of the article explaining why it's -4:56)
In Home Assistant 2021.6 we're going to switch to python-dateutil. You will need to upgrade your custom integration if it relies on the unofficial interface my_time_zone.localize(my_dt). Use Python's official method my_dt.astimezone(my_time_zone) instead.
The property hass.config.time_zone will also change to a string instead of a time zone object.
Thanks to @bdraco for helping revive this effort and push this change past the finish line. We actually found a couple of bugs during the migration! Also thanks to Paul Ganssle for maintaining python-dateutil and the excellent write up.
Update May 10
Wow, time flies! Paul, the author of python-dateutil and also the author of the blog post that inspired us, pointed us to the fact that Python 3.9 includes upgraded timezone handling and that we should use that instead. With the help of Nick and Paul python-dateutil has been removed again and zoneinfo is used instead (PR).