Fix: python 3.9 does not support | syntax for union

[laurimi]

Running the code on Python 3.9 gives the error:
TypeError: unsupported operand type(s) for |: 'types.GenericAlias' and 'type'

The fix is to use Union and Optional as is done elsewhere in the file.

[filipre]

Thank you very much! Eventually, I'll drop support for Python 3.9 but if this is the only place right now, I'm fine with continuing supporting it.