TST: Add an unfailable CI run against pandas main

[bashtage]

It would probably be a good idea to add a CI run against pandas main. Ideally, this would use nightly wheels to save on time. Not sure how easy this is with poetry and the current CI setup.

[bashtage]

The CI run would be info only, but would make it easier to see any upcoming changes. It would be green for pass, and yellow for fail, but never red.

[Dr-Irv]

Agree this is a good idea. PR welcome!

[twoertwein]

Not sure how easy this is with poetry and the current CI setup.

Could simply give poe pytest a --nightly option, similar to stubtest #184