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
Uses a stripped down, updated variant of like Pulsar/Planemo/ephemeris Makefile stuff. Modified in dozens of ways for tags, version and path differences, etc.. Symbolic links to keep everything in line.