Cleanup python setup for dune 2.9 and newer

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.

Before you request a review from someone, make sure to revise the following points:

• does the new code follow the style guide?
• do the test pipelines pass? (see guide on how to run pipelines for a merge request)
• is the code you changed and/or the new code you wrote covered in the test suite? (if not, extend the existing tests or write new ones)
• does your change affect public interfaces or behavior, or, does it introduce a new feature? If so, document the change in CHANGELOG.md.
• is the list of the header includes complete? ("include what you use")
• all files have to end with a \n character. Make sure there is no \ No newline at end of file comment in "Changes" of this MR.