rename master branch to main

[dlyongemallo]

I work on a number of GitHub projects and the majority use main as the name of the primary branch. This project and PyZX are the exceptions. Since PyZX is used by some of these other projects, I switch between them and PyZX quite frequently and I mistype main instead of master or vice versa often enough to be annoying. So i filed zxcalc/pyzx#221 against PyZX.

For consistency, I'm filing the same issue here (since obvious ZXLive is one of the projects where I flip between it and PyZX while reading and developing the code).