You'd be better off just adding your function directly to the class:
def hmm(self):
return 'ok'
requests.models.Response.hmm = hmm
This works just fine:
>>> import requests
>>> def hmm(self):
... return 'ok'
...
>>> requests.models.Response.hmm = hmm
>>> r = requests.get('http://bbc.co.uk')
>>> print r
<Response [200]>
>>> r.hmm()
'ok'
>>> requests.__version__
'1.0.4'