Docs: License

[JacobCoffee]

Summary

Should we use PYTHON SOFTWARE FOUNDATION LICENSE VERSION 2?

[hugovk]

To change, I believe we'd need to get permission from all ~146 contributors to relicense their copyrighted contributions, especially as I don't think they have signed the PSF CLA.

We've done something similar at python/release-tools#249, but there were only ~15 contributors to ask.