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
Split test/base into lib/galaxy_test/base and lib/galaxy_test/driver. Dividing things on whether they import galaxy-app code or not - with the goal of a galaxy-test-base package that doesn't depend on galaxy-app and a galaxy-test-driver package that does.
I've been refactoring around this division for a long time. Certain tests (namely the integration tests) always need to bootstrap Galaxy - but the API tests and the Selenium tests should be installable and runnable without Galaxy in the Python environment. So future galaxy-test-api and galaxy-test-selenium packages should have only an optional dependency on galaxy-test-driver.
Properly sturcturing these namespaces is a precursor for real packages on PyPI, clean separation of the tool shed testing code, and cleans up all the flake8 noqa hacks in test/.
John Chilton