github: Only test on Pytohn 3.6 and 3.9
Speed up the checks and hopefully avoid what seems to be github throttling in the number of jobs.
GitLab Maintenance scheduled for Thursday, 18th December 2025, from 7:00 AM to 10:00 AM BST. GitLab will be in read-only mode and experience short periods of complete downtime (up to 20 minutes) during this window.
Speed up the checks and hopefully avoid what seems to be github throttling in the number of jobs.