text: Title: module 'asyncio' has no attribute 'run' (In python3.6)
  Body: In setup.py, python version can be 3.6/3.7,but it raised exception when i run examples/demo_tcp.py used python=3.6.5.
  ```
  File "demo_tcp.py", line 54, in <module>
      asyncio.run(main())
  AttributeError: module 'asyncio' has no attribute 'run'
  ```
  Actually, asyncio.run is a Python 3.7 addition