Update redis requirement from <4.6,>=3.5 to >=3.5,<5.1

Updates the requirements on [redis](https://github.com/redis/redis-py) to permit the latest version.
- [Release notes](https://github.com/redis/redis-py/releases)
- [Changelog](https://github.com/redis/redis-py/blob/master/CHANGES)
- [Commits](https://github.com/redis/redis-py/compare/3.5.0...v5.0.0)

---
updated-dependencies:
- dependency-name: redis
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

 django-environ>=0.4,<0.11
 django-redis>=4.12,<5.3
 pysolr>=3.9,<3.10
-redis>=3.5,<4.6
+redis>=3.5,<5.1
 requests>=2.25,<3
 uWSGI>=2.0.19
 whitenoise>=5.2,<6.5