ResourceWarning: unclosed socket.socket when I run a unittest？

requests show me following warning when I run a unittest:
```python
ResourceWarning: unclosed <socket.socket fd=5, family=AddressFamily.AF_INET, 
type=SocketKind.SOCK_STREAM, proto=6, laddr=('219.82.68.148', 36006), raddr=('125.46.22.55', 80)>
```

But this warning won't happen if I don't run the code in a unittest environment.

Is there somebody who can tell me why this warning happen?