Bump minimum Python to 3.9

[jacobtomlinson]

Dask dropped 3.8 support in dask/community#315. This PR makes 3.9 the minimum version here.

[jacobtomlinson]

Dask Jobqueue uses container images in CI that were built by the latest run on main. So int his case those images still use 3.8. In this PR I changed things to build instead of pull to ensure things work with 3.9.

Everything is passing except for the known SGE test issue (see #653) so I'm confident with the changes here. I'm going to revert the builds back to pulls and merge which should get things happy again once main is built.

[jacobtomlinson]

Looking at the times here it actually only took a couple of minutes to build the containers. Pull an image built from a previous branch feels a little funny and causes a headache in PRs like this. So I'm going to leave the builds in here instead of the pulls. We can always revert this in the future if it becomes a problem, or explore caching the images in a different way.