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.

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)