Fix test for Python 3.9

Author: RutgerRauws

notubiz/api/_converter.py

@@ -4,7 +4,6 @@
 from notubiz.api._helpers import parse_date
 from notubiz.api._hooks_registry import Hooks
 
-@staticmethod
 def get_converter() -> Converter:
     converter = Converter()
     converter.register_structure_hook(datetime, lambda date_string, _: parse_date(date_string))