Typing breaks python 3.8 support

The syntax used in https://gitlab.dkrz.de/hsm-tools/pyslk/-/blob/master/pyslk/constants.py#L29 is failing with python 3.8. This syntax has only been introduced in python 3.9 (https://docs.python.org/3/whatsnew/3.9.html#type-hinting-generics-in-standard-collections).