[BUG] Compile agama on CI and RTD

[nstarman]

Description

Cannot install agama on GitHub and RTD VMs for testing optional dependencies

See also GalacticDynamics-Oxford/Agama#8

Expected behavior

To be able to pip install agama and have the c code compile

Actual behavior

It fails. See GalacticDynamics-Oxford/Agama#8

Steps to Reproduce

Add agama to optional dependencies in setup.cfg and push to GitHub.

Checklist

• Check out our contributing guidelines.
• Check out our code of conduct.
• Search the GitHub repository to see if a similar issue has already been posted. If a similar issue is closed, have a quick look to see if you are satisfied by the resolution. If not please go ahead and open an issue!
• Please check that the development version still produces the same bug. You can install development version with pip install git+https://github.com/astropy/astropy command.