fix python3

Author: dt3310321

qcloud_cos/cos_client.py

@@ -2496,7 +2496,7 @@ def upload_file_from_buffer(self, Bucket, Key, Body, MaxBufferSize=100, PartSize
             MAXQueue = 1
         pool = SimpleThreadPool(MAXThread, MAXQueue)
         while True:
-            if data == "":
+            if not data:
                 break
             pool.add_task(self._upload_part_from_buffer, Bucket, Key, data, part_num, uploadid, lst)
             part_num += 1