TST: making lowest-deps envs reproducible

I'm seeing errors on reproject + Python 3.11 that I'm willing to bet are effectively noise, assuming
1. this job is really meant to check that astropy-stable + reproject-stable still passes tests on a controlled env with old versions of their combined dependency tree.
2. the error is just showing with recent enough versions of numpy

If my understanding is correct, then the solution would be to make this, and similar jobs run on *reproducible* environments, where everything is resolved to the lowest (oldest) compatible version, with the exception of reproject and astropy themselves (still resolved to latest versions). This is up my alley, as I already did this for astropy's own CI already, but does require a bit of coordination.
In particular, I need confirmation that I'm interpreting the scope correctly. I will also give #44 priority over any attempts to modify how tests are run here.

xref https://github.com/astropy/astropy/issues/18782

Start by reviewing the definitions for this lowest-dependencies job and the similar jobs mentioned in the issue, then read the linked astropy issue for context. Confirm the intended scope with maintainers before changing test execution. Done means the relevant environments resolve reproducibly to the lowest compatible dependency versions while astropy and reproject remain at their latest versions.

Written by the indexing model from the issue text.