Use the logger provided as BeetsPlugin._log, fixes parallel imports crashing...
...on a log level assertion in recent beets versions. Originally, this
fix was committed in 2016, and I don't quite remember the circumstances
crashes happened under. However, looking at the code for beets.plugins,
beets is clearly expecting plugins to use the logger provided through
the plugin class. Thus, it seems this change should be applied whether
or not these crashes could still occur.
Second in a series of PRs to backport changes from wisp3rwind/beets-alternatives to this repo