Swap repo default branch to `main`

[LaikaN57]

@alienbrett can we swap the default branch from master to main as is the new industry standard.

This is under https://github.com/alienbrett/PyAlly/settings > General > Default branch.

If you want to upgrade my role, I can help maintain some of these types of settings as well over time. (I also plan to make some more PRs/settings changes with regards to dependabot and community management.)