Migrate from pytz to zoneinfo

## Summary

The `pytz` package is deprecated. Migrate to `zoneinfo` which is in stdlib since Python 3.9.

## Details

- `zoneinfo` is in stdlib since Python 3.9
- Django 5.x dropped pytz support entirely
- Current usage: Search with `grep -r "pytz" cassandra/ tests/`

## References

- pytz deprecation: https://fedoraproject.org/wiki/Changes/DeprecatePytz

## Priority

Medium