Cleanup python setup for dune 2.9 and newer

Merged Mathis Kelm requested to merge cleanup/python-setup-dune-2.9 into master

What this MR does / why does DuMux need it:

Simplifies installation, CI and CMake code by assuming a dune version of 2.9 or newer. Fixes #1225 (closed)

Checking for the existence of the virtual environment is still necessary as dune 2.10 introduces the ability to deactivate it, which is used in our minimal installation pipelines.

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Edited by Timo Koch

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