No attribute `signal.SIGUSR1` on Windows operating system

Running `OpenVPNClient.connect` results in the error 

```python
AttributeError: module 'signal' has no attribute 'SIGUSR1'
```

This seems to be a windows-specific problem as `SIGUSR1` has [availability "unix"](https://docs.python.org/3/library/signal.html#signal.SIGUSR1).

Any chance of adjusting this to make it windows-compatible? 