Type hint. Appveyor test.

Author: PGijsbers

openml/tasks/split.py

@@ -58,7 +58,7 @@ def __eq__(self, other):
         return True
 
     @classmethod
-    def _from_arff_file(cls, filename):
+    def _from_arff_file(cls, filename: str) -> 'OpenMLSplit':
 
         repetitions = None