Fix threading with python.

Several fixes related to threading and Python.

@andreas.dedner: Can you take a look at threadpass.hh in the constructor. It seems that when parameters are accessed inside thread parallel regions a dead lock occurs.

I disabled it for now to make everything work.

How to reproduce?

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• 5067b02b - [bugfix][ThreadPass] make compile in debug mode.

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Needs dune-fem!408 (merged).

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• 5037a1d2 - [cleanup][ThreadPass] Re-enable threaded initialization since dead lock

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• c8bc92b9 - [feature][testdg] Enabled threaded testing.

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@andreas.dedner: Looks like the CI is happy with the fix in dune-fem. One of the python tests is now running threaded.

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• 37399715 - [cleanup][scripts] Consistent use of venv and env variables.

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enabled an automatic merge when the pipeline for 37399715 succeeds

merged

mentioned in commit 38d82030