Use standard library for date and time when possible

[JuneStepp]

pytz dependency is removed. isodate is still needed for ISO durations and missing features in Python's ISO datetime parser.

closes #1442, closes #1420

[Mogost]

This code will not work in Python 3.8
But currently pyproject.toml has

requires-python = ">=3.8"

So python should be 3.8 should be dropped here or this PR should be dependent on another PR with dropping the mentioned support.

[JuneStepp]

This code will not work in Python 3.8 But currently pyproject.toml has

requires-python = ">=3.8"

So python should be 3.8 should be dropped here or this PR should be dependent on another PR with dropping the mentioned support.

What part doesn't work on 3.8? I'm not using zoneinfo, because it wasn't actually necessary.

[Mogost]

@JuneStepp sorry I was wrong.