Add a mechanism to select python version more dynamically

[BenjaminSchubert]

In some cases, we might want to say "I do not care about the version of python used, but need at least python3.X".

We do not currently have a mechanism to do so, as we can either:

1. Select the python version explicitly
2. Use the version dwas was installed as

Being able to find other versions and use them would be beneficial.

For example, dwas itself uses python3.8 for most things, as this is the lowest python version supported, but the documentation actually requires python3.9, and unless we hardcode this, it's not possible to automatically pick a version.