added import check from astropy for LombScargle

[LucaCerina]

I've noticed that Astropy 6.0.0 changed part of the API so now LombScargle periodogram fails to import (see https://docs.astropy.org/en/v6.0.0/changelog.html)

I've included an import check to be sure it doesn't crash. I don't know if astropy 3.0.4 is a hard requirement, but if it's just for LombScargle it can be probably increased to the latest version.