I understand #722 about ValueError: Unable to configure formatter 'console' was patched in #727, and the 5.2.1 changelog described that the error was a resolved issue. The error have shown up in a test suite for dask/dask-gateway with traitlets 5.2.1.post0 though.

Here is a succeeding test with traitlets 5.1.1 and a failing test with traitlets 5.2.1.post0.