Yep, it looks like the mess with the reference cycles in the tracebacks was just a secondary problem: the real culprit was the internal cache keeping everything else alive.

Attached patch just adds clearing encodings._cache and the CPython internal cache to the ExceptionChainingTest.tearDown() method.

The latter is accomplised via gc.get_referrers() and popping the codec's name from any dict remaining in that list after clearing the known caches, so it will actually handle any hidden name based caches on any interpreter.