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
* Extract "section" headers from the toolbox XML file, and put them into the tool XML;
prepare the tool XML for associating "tags" with the tool.
* Build a toolbox XML file based on augmented tool XML files with section identification.