MAINT: Set astropy latest supported version to 4.2

[rrjbca]

Description

As discussed in #482 there is a conflict between speclite 0.13 (latest) and astropy 4.3.x. It is therefore necessary to make astropy 4.2.x our latest supported version until a fix to speclite is released.

Checklist

• [x] Follow the Contributor Guidelines
• [ ] Write unit tests
• [ ] Write documentation strings
• [ ] Assign someone from your working team to review this pull request
• [x] Assign someone from the infrastructure team to review this pull request