[!692] Fix python version detection.

Merge branch 'bugfix/python-version-detector' into 'master'

ref:core/dune-common This MR fixes the mixup of python version detection. As
in the system cmake script the newest python version needs to be listed first.

See merge request [!692]

  [!692]: gitlab.dune-project.org/core/dune-common/merge_requests/692

cmake/modules/DunePythonCommonMacros.cmake

@@ -72,7 +72,7 @@ include(DunePythonTestCommand)
 
 # Update the list of valid python versions, the shipped CMake modules tend to outdate...
 # Mention all those not present in CMake 2.8.12
-set(Python_ADDITIONAL_VERSIONS 3.4 3.5 3.6 3.7)
+set(Python_ADDITIONAL_VERSIONS 3.8 3.7 3.6 3.5 3.4)
 
 # Find the Python Interpreter
 set(_VERSION_STRING "")