devel/py-rq: Update to 2.9.1

• For now, stick with this version, because the newer version, 2.10.0 has introduced changes to the handling of scheduler identities that don't quite work with the current version of py-django-rq.

Changelog since 2.8.0:

https://github.com/rq/rq/blob/v2.9.1/CHANGES.md

Approved by: skreuzer (maintainer, implicit)