Add a Github action that automates the release process as much as possible. Use [mypy-ls's release action](https://github.com/Richardk2n/mypy-ls/blob/master/.github/workflows/python-publish.yml) for inspiration. Update [RELEASE.md](https://github.com/python-lsp/python-lsp-black/blob/255c32608d3f8354a89d9fcf8ab1fd3b78320b1d/RELEASE.md) accordingly.