Fix Unions to Literals

[tekktrik]

Fixes #76, which was using Union where Literal is needed in some places.

[FoamyGuy]

Looks good to me.

Thank you @tekktrik!

I was unaware Literal was it's own thing and those values couldn't be used in other types of operators.

Do you think it would be possible or sensible to have one step in the actions that attempts to import the module within Linux / Blinka context to confirm that it is importable?

I did run this one on a microcontroller during testing, but that doesn't catch an issue like this since the types are ignored there. I hadn't thought to import it on a RasPi to verify, I can start attempting that though.

[tekktrik]

It's one of the things I'd be interested in trying to do in parallel when I pick the rp2040js CI work I was doing again, to have Blinka import the library as you say. I do think we'll need to consider what how things are imported - for example a folder with an empty __init__.py file can be successfully import and make it look like the library overall is good, so we'll need to make sure we import multiple variations to fully check (e.g. import libname and import libname.sublibname).

I really like the idea and think it can be easily implemented in a way similar to: adafruit/actions-ci-circuitpython-libs#19 and it's planned implementation in the CI: adafruit/workflows-circuitpython-libs#23