Make INTERPRETER argument a single-valued parameter

No support for passing multiple interpreters intended.

cmake/modules/DunePythonTestCommand.cmake

@@ -36,8 +36,8 @@ function(dune_python_add_test)
   # Parse Arguments
   include(CMakeParseArguments)
   set(OPTION)
-  set(SINGLE WORKING_DIRECTORY NAME)
-  set(MULTI SCRIPT COMMAND LABELS INTERPRETER)
+  set(SINGLE WORKING_DIRECTORY NAME INTERPRETER)
+  set(MULTI SCRIPT COMMAND LABELS)
   # set(MULTI COMMAND LABELS)
   cmake_parse_arguments(PYTEST "${OPTION}" "${SINGLE}" "${MULTI}" ${ARGN})
   if(PYTEST_COMMAND)