Astropy >= 4.2 will cause us to drop support for Python 3.6.

[emirkmo]

This will cause us to have to drop support for Python 3.6. This will cause problems for some of the systems currently running the automatic parts of the code, until we can get them upgraded. What is the reason for the strict "astropy >= 4.2" requirement?

Originally posted by @rhandberg in #26 (comment)

Let's keep track of this issue here. My nearest neighbor WCS correction backup requires a function in Astropy 4.2. In general, it's good to have access to later Astropy versions as it leaves us more flexible to implement ready-made solutions and functions from them into the pipeline.. However, it's also possible to just copy the one functionality we need and not require Astropy 4.2.

Astropy 4.2 may also be needed for latest version of Photutils (not sure, we should check), which does fix some bugs, I don't know if they are relevant or not.

However, let's hold off this workflow breaking update for now.

[emirkmo]

Also, the latest photutils 1.1.0 needs python 3.7, but photutils 1.0.2 is ok on Pyhthon 3.6. I think for security and compatibility, we should make sure to move onto python 3.7 asap.

[emirkmo]

This is breaking due to the AST machines that we rely on for the pipeline. When we get our own or when the AST machines are updates, let's quickly move to python 3.7+

[emirkmo]

As the AST machines are no longer used, we can go ahead with this and track it on #41