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
- Update links
- Replace disappeared "Try Git" tutorial with Software Carpentry's one
- Include Python style guidelines and testing instructions from the Hub