Collapse `master`/`develop` branches into `main`

[chrisjsewell]

As we've done for aiida-core etc, it feels overkill to bother with gitflow for this package, and easier to delopy new releases without

@ltalirz @dev-zero?

[ltalirz]

I have no issue with switching to main - feel free to go ahead and make the change

[chrisjsewell]

done