PEP 604: support for Union types a 8000 s X | Y

This will be added as a feature in Python 3.10. I actually thought this was already in the enhanced type annotation section, but noticed coconut currently only does this kind of thing for optionals and callables.

Would definitely love to see this new syntax added to coconut, especially as I've been writing more and more pydantic data-structures. One step closer to "elm-as-python" :D