Infrastructure updates notice from Astropy Project

[pllim]

Hello from Astropy Project!

The following updates to your package might be necessary to ensure compatibility with the latest stable version of Astropy:

• [x] MPLBACKEND is now set to Agg in ci-helpers, packages expecting interactive plotting should override it in .travis.yml
• [ ] Astropy 3.1 is not compatible with Numpy <1.13 versions. If you want to keep testing these older Numpy versions, please use ASTROPY_VERSION=3.0 or ASTROPY_VERSION=LTS in your Travis CI matrix.
• [x] Add sphinx-astropy as a package dependency if you are using astropy-helpers 3.1 or later. Otherwise, your documentation build (e.g., on ReadTheDocs) might fail.
• [x] If you are using six that is bundled with Astropy, please consider using the standalone six package instead.

If these are no longer applicable to you, please close the issue.

This is an automated issue for packages that opted in for automated astropy-helpers update. If this is opened in error, please let @pllim know.

xref astropy/astropy-procedures#108

[Cadair]

I think we are good on this right?

[bsipocz]

No we're not, the travis matrix is still a mess.