Python version support strategy

[damskii9992]

General

As new python versions come out and are added to our builds, we cannot continue supporting older versions of python due to the necessary maintenance burden. As such it is necessary to have a strategy for when to drop support for older versions of python/when to adopt new versions.

Limited by our dependencies

As our main dependency is Scipp, it makes sense to adopt their support strategy when possible, which is always the latest 3 versions of python.

It is important to also consider our other dependencies, such as PySide and pyinstaller, in case they block adoption of newer python versions.

Link to the original discussion: #26