Fix the following errors reported by flake8-bugbear 21.4.3:
B017 assertRaises(Exception): should be considered evil. It can lead to
your test passing even if the code being tested is never executed due to a
typo. Either assert for a more specific exception (builtin or custom), use
assertRaisesRegex, or use the context manager form of assertRaises.
Upgrade syntax using `pyupgrade --py36-plus` .
Manually drop several `six` imports.
Also:
- Remove broken pr_cache in scripts/bootstrap_history.py
- Fix broken prefix removal in lib/galaxy/tool_util/deps/mulled/mulled_build.py