Pretty much completely rewritten theme with custom HTML translator and a
few parts of the old theme extracted to their own extensions.
Banner images thought not to be that huge after all, and not worth the
hassle of them living in a different repository.
co-authored with @stefanorigano
* document and warn that checks and fast_suite in tests sub-packages are
deprecated and have no effect
* avoid iterating all currently loaded modules when looking for test
modules in a tests sub-package
* replace use of __import__ by importlib
Fixes#3152