nit: link in doc opening blurb no longer functional

[twirrim]

Super quick nit thing:
From https://tenacity.readthedocs.io/en/latest/

It originates from [a fork of retrying](https://github.com/rholder/retrying/issues/65) which is sadly no longer [maintained](https://julien.danjou.info/python-tenacity/).

That second link, for "maintained", doesn't work any more. I don't see an obvious replacement for it after searching the blog, or I'd have tried to do a simple PR.