Use `ZoneInfo` instead of dateutil to handle time zones.

[h-mayorquin]

Now that python 3.9 is minimal have you considered using ZoneInfo from the standard library instead of dateutil?

https://docs.python.org/3/library/zoneinfo.html

Eventually, you can eliminate another dependency and also there might be undesirable inconsistencies between the standard library approach and other libraries that would be good to avoid.

[rly]

I support this.

[oruebel]

According to the ZoneInfo docs, one issue we may need to consider is

"This module does not work or is not available on WebAssembly. See WebAssembly platforms for more information."

Not sure which packages use h5wasm, but we may need a fallback import for that case.

[rly]

Also worth noting from the zoneinfo docs:

The zoneinfo module does not directly provide time zone data, and instead pulls time zone information from the system time zone database or the first-party PyPI package tzdata, if available. Some systems, including notably Windows systems, do not have an IANA database available, and so for projects targeting cross-platform compatibility that require time zone data, it is recommended to declare a dependency on tzdata. If neither system data nor tzdata are available, all calls to ZoneInfo will raise ZoneInfoNotFoundError.

PyNWB does not really use timezone data except in tests, docs, and a couple places in the core code that could be replaced with datetime methods nowadays, I believe.