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
merge master -D org: PR 181 (Try to fix intermittent SSL error in RebaseTest.has_rebased_pr())
merge master -D org: PR 186 (Comment on possible conflicts)
merge master -D org: PR 187 (scc check-prs caching)
merge master -D org

Repository: openmicroscopy/snoopycrimecop
Already up-to-date.

Merged PRs:
  # PR 181 sbesson 'Try to fix intermittent SSL error in RebaseTest.has_rebased_pr()'
  # PR 186 manics 'Comment on possible conflicts'
  # PR 187 sbesson 'scc check-prs caching'

Generated by SCC-merge#400 (http://ci.openmicroscopy.org/job/SCC-merge/400/)