Prepare for v3 release

Some WIP improvements we could bring from upstream:

- https://github.com/litl/backoff/issues/93
- ~https://github.com/litl/backoff/pull/103~ - https://github.com/python-backoff/backoff/pull/50
- https://github.com/litl/backoff/pull/130/
- ~https://github.com/litl/backoff/issues/148 (publish them!)~ - https://github.com/python-backoff/backoff/pull/39
- https://github.com/litl/backoff/pull/204