fix redis-py install constraint

[aaroncoville]

Summary

setup.py caps redis<5, blocking redis-py 4.6.0 and 6.1.0 (both in CI) and any downstream using redis>=5.

Fix: widen to redis>=4.0,<7. Bump version to 0.25.1. Add CHANGELOG entry. No runtime changes.

Bug Repro

pip install tasktiger==0.25.0 redis==6.1.0
# ERROR: ResolutionImpossible

<5 cap forbids 6.1.0 — same version requirements.txt pins and CI tests.