It was added back in bfc408b probably due to a misunderstanding of
https://docs.python.org/3/howto/logging.html#configuring-logging-for-a-library.
The default behaviour of logging WARNING and above is good and
desirable (hiding unhandled exceptions in update handlers by default
was a big, accidental mistake). NullHandler is used to *prevent*
this good default, so it shouldn't be used in the first place.
0997e3fa9f