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
- Replace test using Cut1 with two tests each using framework test tools loaded with Selenium tests by default.
- Create a symbolic link so that tour generator is configured with default test webhooks.
- Establish and use components and navigates_galaxy.py abstractions for tour generation and navigation.
- In one of the tool tests actually walk the tour (previously we only verified the initial tour popover).
- Populate test server with demo webhooks by default.
- Eliminate special logic needed to treat the demo directory differently.
- Document how to enable the demo webhooks in galaxy.ini.sample.
- Populate the demo webhooks when starting Galaxy with GALAXY_RUN_WITH_TEST_TOOLS=1 set.