AuthHandler example in docs shows wrong code

[stphnthiel]

This docs page on AuthHandlers misses one line of code to be working

import requests
from requests_toolbelt.auth.handler import AuthHandler

def gitlab_auth(request):
    request.headers['PRIVATE-TOKEN'] = 'asecrettoken'
    return request --> add this line!

handler = AuthHandler({
    'https://api.github.com': ('sigmavirus24', 'apassword'),
    'https://gitlab.com': gitlab_auth,
})

session = requests.Session()
session.auth = handler
r = session.get('https://api.github.com/user')
# assert r.ok
r2 = session.get('https://gitlab.com/api/v3/projects')
# assert r2.ok

The current example gives the following error:

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Input In [17], in <cell line: 1>()
----> 1 r = session.get("some-url")

File python3.8/site-packages/requests/sessions.py:555, in Session.get(self, url, **kwargs)
    547 r"""Sends a GET request. Returns :class:`Response` object.
    548 
    549 :param url: URL for the new :class:`Request` object.
    550 :param \*\*kwargs: Optional arguments that ``request`` takes.
    551 :rtype: requests.Response
    552 """
    554 kwargs.setdefault('allow_redirects', True)
--> 555 return self.request('GET', url, **kwargs)

File python3.8/site-packages/requests_toolbelt/sessions.py:64, in BaseUrlSession.request(self, method, url, *args, **kwargs)
     62 """Send the request after generating the complete URL."""
     63 url = self.create_url(url)
---> 64 return super(BaseUrlSession, self).request(
     65     method, url, *args, **kwargs
     66 )

File python3.8/site-packages/requests/sessions.py:528, in Session.request(self, method, url, params, data, headers, cookies, files, auth, timeout, allow_redirects, proxies, hooks, stream, verify, cert, json)
    515 # Create the Request.
    516 req = Request(
    517     method=method.upper(),
    518     url=url,
   (...)
    526     hooks=hooks,
    527 )
--> 528 prep = self.prepare_request(req)
    530 proxies = proxies or {}
    532 settings = self.merge_environment_settings(
    533     prep.url, proxies, stream, verify, cert
    534 )

File python3.8/site-packages/requests/sessions.py:456, in Session.prepare_request(self, request)
    453     auth = get_netrc_auth(request.url)
    455 p = PreparedRequest()
--> 456 p.prepare(
    457     method=request.method.upper(),
    458     url=request.url,
    459     files=request.files,
    460     data=request.data,
    461     json=request.json,
    462     headers=merge_setting(request.headers, self.headers, dict_class=CaseInsensitiveDict),
    463     params=merge_setting(request.params, self.params),
    464     auth=merge_setting(auth, self.auth),
    465     cookies=merged_cookies,
    466     hooks=merge_hooks(request.hooks, self.hooks),
    467 )
    468 return p

File python3.8/site-packages/requests/models.py:320, in PreparedRequest.prepare(self, method, url, headers, files, data, params, auth, cookies, hooks, json)
    318 self.prepare_cookies(cookies)
    319 self.prepare_body(data, files, json)
--> 320 self.prepare_auth(auth, url)
    322 # Note that prepare_auth must be last to enable authentication schemes
    323 # such as OAuth to work on a fully prepared request.
    324 
    325 # This MUST go after prepare_auth. Authenticators could add a hook
    326 self.prepare_hooks(hooks)

File python3.8/site-packages/requests/models.py:559, in PreparedRequest.prepare_auth(self, auth, url)
    556 r = auth(self)
    558 # Update self to reflect the auth changes.
--> 559 self.__dict__.update(r.__dict__)
    561 # Recompute Content-Length
    562 self.prepare_content_length(self.body)

AttributeError: 'NoneType' object has no attribute '__dict__'

Including that here so people find it when Googling.