Add Python 3.9 back temporarily for a 24.08 hotfix

[raydouglass]

We're planning one final hotfix for 24.08 which supports Python 3.9, so add it back here out an abundance of caution. Without this, the Python 3.9 builds will use outdated images.

This can undone in ~2 weeks.

[jameslamb]

Looks like you're running into the same ci-conda build issues I've been seeing on other PRs here this morning.

I'm tracking that in #185 and I'm using #183 to investigate.

[jameslamb]

@raydouglass I put up a hack to temporarily fix CI in #186 . It could be merged to unblock CI here for the purposes of rolling out this 24.08 hotfix if you need something quick.

[raydouglass]

@raydouglass I put up a hack to temporarily fix CI in #186 . It could be merged to unblock CI here for the purposes of rolling out this 24.08 hotfix if you need something quick.

Thanks @jameslamb! This is not urgent and can wait a couple days for a solution.

[jameslamb]

Thanks @raydouglass ! We got #186 to a point where it now contains a long-term solution and not a hack 😁

Once I merge that to main, I'll update this PR and merge it for you if you'd like.

[jameslamb]

Thanks for the review @msarahan !

@raydouglass I opened #187 to track the work of reverting this in a few weeks.