rrs_upstream_history.py (and rrs_distros.py) can fail before doing anything, e.g.:

  File ".../lib/bb/cooker.py", line 15, in <module>
    from bb import multiprocessing
  ImportError: cannot import name 'multiprocessing' from 'bb'

load_recipes() in rrs/tools/common.py imports bb.fetch before recipeparse.init_parser() checks out the BitBake revision for the branch, so the bb package comes from whatever revision is checked out at that moment, e.g. an old one left by rrs_upgrade_history.py, which checks out BitBake revisions from the past. The rest of BitBake is then loaded from the checked-out revision, and they don't match.

Fix: import bb.fetch after init_parser() has checked out BitBake.