patch takes kwargs, just pass headers = {your_header}:

def patch(url, data=None, **kwargs):
    """Sends a PATCH request.

    :param url: URL for the new :class:`Request` object.
    :param data: (optional) Dictionary, bytes, or file-like object to send in the body of the :class:`Request`.
    :param \*\*kwargs: Optional arguments that ``request`` takes.
    :return: :class:`Response <Response>` object
    :rtype: requests.Response
    """

    return request('patch', url,  data=data, **kwargs)

So something like this:

head = {"Authorization":"Token token=xxxxxxxxxxxxxxxxxxxxxx"}
url = 'http://0.0.0.0:3000/api/v1/update_experiment.json'
payload = {'expt_name' : 'A60E001', 'status' : 'done' }

r = requests.patch(url, payload, headers=head)

params should always be a dict or a sequence of 2-value tuples, which are then encoded for you. You, however, want to upload an already encoded body, so you want the data keyword instead:

requests.patch(url, data=json.dumps(payload), headers=headers)

In fact, data is the second parameter, so you could even do:

requests.patch(url, json.dumps(payload), headers=headers)

as you normally only send opaque data with PATCH. The .post() and .put() methods behave in the same way, the second parameter is the data keyword.