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
instead of converting the Markdown to reStructuredText with pandoc.
Fix the broken tables, but code inside tables is not coloured as
in the rest of the document.
Generate intermediate rst file instead of html
Move generation to doc/Makefile
Use pandoc to generate table of contents and fragment ids.
Remove nested inline markup (not supported by reStructuredText)
Fix https and other links.