fix redis-py install constraint

[aaroncoville]

Summary

setup.py pins redis>=3.3.0,<5, which excludes every redis-py version the CI matrix actually tests (4.6.0, 6.1.0) and breaks co-install with any downstream that depends on redis>=5. This PR widens the bound to
redis>=4.0,<7, bumps the version to 0.25.1, and adds a CHANGELOG entry.

Metadata-only — no runtime code changes.

Reproducing the bug

# In a fresh python 3.10+ environment:
pip install tasktiger==0.25.0 redis==6.1.0

ERROR: Cannot install redis==6.1.0 and tasktiger==0.25.0 because these package versions have conflicting dependencies.
ERROR: ResolutionImpossible

The <5 cap in 0.25.0 forbids redis-py 6.1.0, which is the version
requirements.txt pins and that CI tests against.