Set up compat helper in this repo

[sefffal]

This package is stuck on an older version of FITSIO, which is pretty quick to fix.
but we should set up compathelper to catch this faster in future.

[giordano]

CompatHelper is already there, but it probably stopped for inactivity.

[sefffal]

Ah got it!
I’ll just wait for CI to complete and then I’ll merge the compat helper PRs.