Merge pull request #3041 from ThomasWaldmann/stderr-flush-1.1

 flush json mode progress stderr output (1.1-maint)

src/borg/helpers.py

@@ -1444,7 +1444,7 @@ class ProgressIndicatorBase:
             finished=finished,
             time=time.time(),
         ))
-        print(json.dumps(kwargs), file=sys.stderr)
+        print(json.dumps(kwargs), file=sys.stderr, flush=True)
 
     def finish(self):
         if self.json: