with:
```
find packages/ -name '*.py' | xargs black
```
The `packages/` directory is currently excluded in the `[tool.black]`
section of `pyproject.toml` because black follows symlinks.
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