Verify Mac functionality

[Teschl]

Verify that the package can be installed and build on mac.

[wkearn]

The easiest way to make sure stuff works across operating systems is to run a CI job through GitHub Actions, which can run on Linux, Windows and Mac. We've got one for libtopotoolbox, but I'll set one up for pytopotoolbox.

[wkearn]

Everything works on Mac on CI.

I guess we still need to update the installation instructions. I think they are probably more or less identical to the Linux ones. I do have access to a relatively old Mac at home that I can check things on.