Remove `pytz`; replace with IANA Time Zone Database

[Zac-HD]

The overview page prominently lists pytz:

https://github.com/mahmoud/calver/blob/f0ccce2ca238d3260a0ee7c1f4cfca1a3e99bdd8/entries/overview.md?plain=1#L160-L177

However, pytz has serious compatibility issues with Python standards, and has largely been replaced by the zoneinfo module in the standard library.

I suggest minimally adapting this section to refer directly to the tz database, which describes the versioning scheme:

Since 1996, each version has been a four-digit year followed by lower-case letter (a through z, then za through zz, then zza through zzz, and so on).

[mahmoud]

Hey Zac! That's a great idea. No special loyalty to pytz here. I'll make the change, unless you wanted to submit a PR.

[Zac-HD]

Sounds fun, I'll send you a PR tonight!