Remove support for py 3.6

[emirkmo]

support for py 3.6 needs to be dropped. It's missing many core features like static typing etc. I ideally actually want to move to 3.9+ for the pipeline part but we should be able to support 3.7/3.8 by using some libraries like typing or from __future__... imports, as necessary.

As a note, astropy v5.0+ only supports python 3.8+, so we should at least get parity there soon.

TODO

• Remove py 3.6 from github actions
• Update tests.
• Remove any references to py 3.6, docs, tests, etc.
• Decide on whether to remove py 3.7 as well.

[emirkmo]

#27 Looks like we noticed this in a previous issue. I think we should pull the trigger on this now and make sure nerdrage is running the latest python possible as well (3.9 or 3.10 ideally).