Update minimum supported python to 3.9

[ljgray]

Python 3.8 is reaching end-of-life in October. I think we're OK to update this one, but we should be careful with deploying since some machines might be running python <= 3.8

[ketiltrout]

I hesitate a bit about this because of ch_util's heavy use in the acquisition system, but I think it's still probably reasonable.

[ketiltrout]

Given that ch_util is going to pull in caput anyways, keeping this one at 3.8 isn't going to help us much if we're re-deploying stuff.

[ljgray]

That's a good point about caput (and driftscan). I guess we should review exactly where these repos are being used and see where we can update python