DEBUGlevel whilst developing plug-ins, Pyblish can tell you a few things about what is going on.
%(message)sis one of the many variables available during formatting, see here for a full list.
basicConfigwill register a handler too, and it's possible to have many. See
log.handlersfor the ones currently registered, it's a plain Python
listthat you can clear to get rid of them.
basicConfigunder-the-hood at startup. Which means that you always see these messages whether you want to or not. You can remove handlers, and edit them despite this.