[python][ci] Run flake8 on dumux python module and test in ci

.flake8

+[flake8]
+max-line-length = 100
+per-file-ignores =
+    # imported but unused and unable to detect undefined names
+    __init__.py: F401, F403
\ No newline at end of file

.gitlab-ci/default.yml

@@ -42,13 +42,14 @@ black (python):
     - black --check --verbose -- test/python
 
 
-pylint (python):
+pylint-flake8 (python):
   stage: linting
   script:
     - source bin/testing/ci-setup-python-env.sh
     - |
       if [ -d build-cmake/python/dumux ] ; then
         pylint --rcfile=.pylintrc build-cmake/python/dumux
+        flake8 build-cmake/python/dumux
       fi
   needs:
     - job: configure