net-mgmt/netbox: Relax requirements for py-redis

• With py-redis 8.0, the default protocol is changed from RESP2 to RESP3 (= REdis Serialization Protocol), which causes problems with Redis < 6.0.
  
  Although Redis 5.x is officially supported up to NetBox version 4.6.10 it has been End-of-Life for a long time and is no longer available in the ports tree.

• Therefore, based on these circumstances, adjust the upper limit for py-redis preemptively so as not to unnecessarily block its updates to the next major version.