Apply the SunPy package template

[namurphy]

SunPy has created a package template and is going through the process of applying the template to the packages it maintains. They're also setting up the infrastructure so that when the template gets updated, then automated pull requests get created to do the updates.

The main benefits of applying this template are helping with software sustainability:

• The long-term maintenance burden will be reduced.
• After funding for XRT ends, using the package template would simplify the process of having SunPy potentially take over XRTpy maintenance.

An example pull request to apply this template to sunkit-instruments is sunpy/sunkit-instruments#133.

Thanks to @nabobalis and @wtbarnes for suggesting this!

[namurphy]

@nabobalis mentioned that one sticking point is that applying this template now means that we would have to switch back to tox from nox. This is because the OpenAstronomy workflows do not currently support nox.

[nabobalis]

We/I can work on adding that, in theory it shouldn't be "hard".