socket.detach implementation has issues

The socket.detach implementation sets the underlying C# socket to `null`, but none of the code is set up to deal with it, this causes most methods to fail with `SystemError`. For example:
```py
import _socket
s = _socket.socket()
s.detach()
assert s.fileno() == -1
```
fails with:
> SystemError: Object reference not set to an instance of an object.