rename master branch to main

[dlyongemallo]

I switch between developing in PyZX and other projects (e.g., the Qiskit ZX transpiler that I'm working on) which use PyZX that have main as the name of the primary branch rather than master. The default branch name was changed in GitHub a number of years ago to the latter. See here.

It would be helpful to update PyZX to use the current standard because of muscle memory, scripts, command-line history, etc. Every time I switch between PyZX and another project I have to readjust.