BLD: build is currently broken on Python 3.8

[neutrinoceros]

415 broke test jobs on Python 3.8 (numpy 1.25 requires Python 3.9 or newer). There are two possible approaches to fix this:

• use oldest-supported-numpy on Python 3.8 instead (keep using numpy>1.25 for other versions)
• drop support for Python 3.8

I'm happy to do the work either way, just please let me know what's preferred here! Ping @astrofrog

[astrofrog]

We should just drop 3.8 in line with NEP29

[astrofrog]

This is already done in https://github.com/astropy/reproject/pull/411

[neutrinoceros]

Awesome. @astrofrog please let me know if you need (or would like) my review. I can make some time for it tomorrow !

[astrofrog]

@neutrinoceros - feel free to review!