Add python 3.9
Python 3.9 has been released: https://docs.python.org/release/3.9.0/whatsnew/3.9.html
And Python 3.5 maintenance has ended: https://www.python.org/dev/peps/pep-0478/#id4
This instance will be upgraded to Heptapod 1.3.1 (GitLab 16.9.3) on 2023-03-28 at 17:00 UTC+1
Python 3.9 has been released: https://docs.python.org/release/3.9.0/whatsnew/3.9.html
And Python 3.5 maintenance has ended: https://www.python.org/dev/peps/pep-0478/#id4