Upgrade Python versions in site banner

Comment from here: https://github.com/buildbot/buildbot/issues/4439#issuecomment-1499052595

> Alternatively, is there a reason Python 2.7 (and 3.5-3.7) support is still shown in the site banner? 3.7.0, the PyPI version also listed there, lists support for only 3.7-3.9 (and I'm assuming it supports newer (as we're using it up through CPython 3.12) and the tags are missing).

https://user-images.githubusercontent.com/17051931/230391129-6b4f5847-0f38-4255-bd4a-c4c6bbf0acd6.png