Not actually testing on MacOS/Windows

[WhiffleFish]

All CI badges point to the same link.

We're only actually testing on ubuntu-latest as made evident in CI.yml.

[zsunberg]

My bad

[zsunberg]

We should definitely not lie on the README, but there are two solutions:

1. actually test on all OSs
2. just have one badge

I don't think there is any OS-specific code, so maybe we would be better citizens with solution (2) not taking up the resources. Also, at least in the olden days, testing on Windows and Mac was super annoying because it would take a long time for a machine to become available. On the other hand, people might be more confident in using it if they see that their OS has a green box next to it (though if they know what the green box means, they probably realize that it doesn't matter).

[rejuvyesh]

I think it's easy enough to test on multiple os with: runs-on: ${{ matrix.os }} and setting up the matrix.os appropriately. IMO we should test on the major 3 OS.

[zsunberg]

This was fixed with #486