Remove __nonzero__ method from models.Response

[sobolevn]

__nonzero__ was used in python2 as we now use __bool__. It is not used at all in python3.
And since setup.py says that reqeusts supports only python >= 3.7, I don't think that it is really required.

I found this while working on typeshed types for requests: https://github.com/python/typeshed/blob/3fe1f5d6c46b39b8b4632aadc70477a729241d25/stubs/requests/requests/models.pyi#L107

[nateprewitt]

Hi @sobolevn,

Thanks for the PR, but I think we’re unlikely to remove this. The use of super().__nonzero__() to implement __bool__ in subclasses was quite popular when the Python 3 migration started. There’s a high chance this will be breaking if we remove the function entirely.