BLD: Set astropy latest supported version to 4.3 and speclite minversion to 0.14

[rrjbca]

Description

speclite 0.14 will contain a fix that allows us to support astropy 4.3.x. Resolves #482

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

[rrjbca]

Right, and they won't until the new version of speclite is released. But you can still approve if you are satisfied that the changes are accurate.