Upgrade or replace ics library (incompatible with tatsu 5.8+)

[japsu]

Problem

ics 0.7.2 (released 2022-07-06) uses tatsu internally and passes a buffer_class keyword argument to tatsu's ParserConfig. This parameter was removed in tatsu 5.8, causing a ValueError at import time:

ValueError: Unknown settings for ParserConfig: {'buffer_class'}

Current workaround

tatsu is pinned to <5.8 in pyproject.toml to keep ics working. This blocks upgrading tatsu and may conflict with other dependencies over time.

Desired outcome

Either:

1. Upgrade ics — if a newer release fixes tatsu compatibility (as of 2026-03-21, the latest stable is still 0.7.2)
2. Replace ics with a maintained alternative (e.g. icalendar) if ics remains unmaintained

References

• ics PyPI: https://pypi.org/project/ics/
• tatsu changelog: https://tatsu.readthedocs.io/en/stable/CHANGELOG.html