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
str.join()
by using generator expressions and tuples instead of lists.
better job of catching them in the application, but this is good enough for the moment.