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
The idea behind an 'extended metadata' for datasets is to provide a mechanism to encode
and index user generated metadata and keep it attached to files. Users can provide a
JSON style data structure which will then be indexed and stored. Later this index can
be used to search and discover datasets via a pseudo-formal schema.
This patch introduces the tables, and makes it possible to inject user metadata into a file
load using the 'extended_metadata' field in the query.