Clear trace when enter set_trace

Author: gaogaotiantian

Lib/bdb.py

@@ -336,6 +336,7 @@ def set_trace(self, frame=None):
 
         If frame is not specified, debugging starts from caller's frame.
         """
+        sys.settrace(None)
         if frame is None:
             frame = sys._getframe().f_back
         self.reset()