Update setup.md with specific actions (if any) required to setup software for the course

[DC23]

Needs to cover the following items:

1. Having a local install of Python 3.5 or newer
   1. Rationale for 3.5+ is so that we can refer to PEP-484 for static type hints in the episodes for functions and testing.
2. Having a text editor (most systems have something already installed, so skip this?)
3. Download the test data files. Note that this can't be finalised until we know what the files are and where they are hosted.

We should also determine if there are common versions of some of this material so we can get a consistent usage across all data school lessons.

[DC23]

I will be writing something soon for the workflow lesson. Could probably merge something from that

[DC23]

Also the python-files setup could be useful