plots --open: not working on macOS

[skshetry]

We use relpath to open files in the browser.

https://github.com/iterative/dvc/blob/59cfc2c4a43e9b9d2fdb583da80d24e82ae39c26/dvc/command/plots.py#L60

This is mostly done to support wsl, and the fact that all other Platform seem to work, if it's relative or absolute.

The reason why it does not work is because web browser.open messes up. 🤦🏼
https://github.com/python/cpython/blob/39dd141a4ba68bbb38fd00a65cdcff711acdafb5/Lib/webbrowser.py#L638-L639