logger: reduce pillow verbosity

With newer pillow versions, pillow would create debug log lines for
every single PNG images it touches, so that spams the log in
Overviewer's verbose mode quite a bit.

overviewer_core/logger.py

@@ -298,3 +298,6 @@ def configure(loglevel=logging.INFO, verbose=False, simple=False):
         logger.overviewerHandler.setFormatter(formatter)
         logger.addHandler(logger.overviewerHandler)
         logger.setLevel(loglevel)
+
+    # Make sure Pillow doesn't spam us in verbose mode
+    logging.getLogger("PIL").setLevel(logging.WARNING)