OK, I have fixed test_trace by tweaking the trace module in default. I have decided to follow Antoine's suggestion-by-question and get test_pydoc working before I merge by getting ImportError spruced up in default but pydoc changed in bootstrap_importlib. I am *not* going to try to get registry-based imports working because I don't have a Windows machine and could quite easily break the build. I would rather have a Windows expert add the support.
IOW:
* Issue #1559549
* Fix pydoc in bootstrap_importlib
* Big patch merged into default
* ???
* Profit!