Add logic to cache rebased PRs using pickle
Remove deprecated  git notes strategy (replaced by caching)
Add checkpoint for directory existence before dumping
Store a set of rebased PRs for caching

- Initialize and empty set rebased_prs
- Update the set with the keys() of the cached links
- Add every rebased PR where both the source and the target are merged
- Remove the mismatching PRs from the rebased set
- Use this set for caching the links
Fix flake8
Add check before removing mismatching PR number from the rebased set
Merge pull request #187 from sbesson/checkprs_cache

scc check-prs caching