Use simple print in tracker print function. (#6609)

python-package/xgboost/rabit.py

@@ -1,7 +1,6 @@
 # coding: utf-8
 # pylint: disable= invalid-name
 """Distributed XGBoost Rabit related API."""
-import sys
 import ctypes
 import pickle
 import numpy as np
@@ -79,8 +78,7 @@ def tracker_print(msg):
     if is_dist != 0:
         _check_call(_LIB.RabitTrackerPrint(c_str(msg)))
     else:
-        sys.stdout.write(msg)
-        sys.stdout.flush()
+        print(msg.strip(), flush=True)
 
 
 def get_processor_name():