Remove pytz dependency

[danielhollas]

Quoting from the pytz README

Projects using Python 3.9 or later should use the standard library zoneinfo module with time zone database updates from tzdata package. pytz offers no advantages beyond backwards compatibility with code written for earlier versions of Python.

The only use is in aiida.common.timezone:timezone_from_name() function so should be hopefully easy to migrate.

I've noticed this in AiiDAlab because aiida-core currently pins on an ancient 2021 pytz version that was generating warnings.