Python tutorials: Allow skipping venv setup

[MakisH]

closes #679

Checklist:

• I added a summary of any user-facing changes (compared to the last release) in the changelog-entries/<PRnumber>.md.

[fsimonis]

The amount of extra shellscript for this is pretty daunting.

I commented on the issue that an environment variable would be way easier to integrate.
If you want to use shellscript for this, then it would be very nice to refactor this into a single file and include it somehow.

[MakisH]

Very good point, an environment variable would make everything easier and would not require input from the user. I will adapt.