introduced in https://github.com/galaxyproject/galaxy/pull/18003 .
In particular, when using requests in the tests to connect to Galaxy
or the ToolShed it's not needed to use the modified headers.
Also:
- Move some imports after conditional imports.
- Fix type annotations in `lib/tool_shed/test/base/populators.py`
- Fix typo bug in `test/unit/util/test_get_url.py`
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