Disable or fix Python tests in CI for release 3.4
The new Docker images enable the Python bindings in the full container (previously untested). The Python bindings are tested if Dune >= 2.8. However due to a configuration issue the tests currently fail to run/compile. Help is on the way in !2681 (merged) but this is not going to be part of 3.4. Therefore we should probably just manually disable the Python tests in the CI config on master+release/3.4. Or if the problem is solved in !2681 (merged) soon, add the fix to the current CI config.