- Remove shelve picture cache as it has had a fair number of historical
issues. Original issue for which it was added should be long
resolved. Additionally this allows additional consolidation of the
various cache code and potentially dbs in the future.
- Remove all related preferences and related code for changing cache
backend between sqlite and shelve.
- Format all files with black
- Update tox.ini flake8 arguments to be compatible
- Add black to requirements-extra.txt
- Reduce ignored flake8 rules and fix a few violations