Add PySR integration test

[MilesCranmer]

Now that PySR has switched to PythonCall.jl, here is the promised integration test :)

I'm installing and running it in a separate virtualenv so as to not interfere with anything.

[cjdoris]

This looks cool thanks! However I don't think integration tests are suitable for running on every commit in regular CI. Instead I think it would make sense to have a separate github workflow for integration tests which is run manually - and I'll ensure to run it before each release.

Are you familiar enough with github workflows to do this or would you rather I did it?

[MilesCranmer]

Sounds good to me – just adding the workflow_dispatch key right?

Also would it be useful to have a daily/weekly cron job or something, to test against main? (but not so regular as to be a nuisance)

[cjdoris]

Yeah a weekly cron job would also be fine.