[MNT] add renovate app

[tschm]

Keep versions of workflows, pre-commit hooks and dependencies up-to-date

[fkiraly]

I think currently dependabot could do most of the work if not all, and dependabot is a GH native functionality.

See here: https://github.com/PyPortfolio/PyPortfolioOpt/pull/689

I would be interested to hear your opinion what the difference in functionality is, and the advantage, of renovate?

[tschm]

Renovate can update the pre-commit hooks, easier to configure, can update within Dockerfiles, can be configured for auto merge, can merge the updates, awesome tool