title: Can't register webhook for Twitter Account Activity API [python3]
body: <p>I'm trying to set up a Twitter app using the Account Activity API, to replace my old set up which used the user streaming endpoint. I want to be able to get DM messages to one user sent to a particular URL in real time.</p>
  
  <p>Following <a href="https://developer.twitter.com/en/docs/accounts-and-users/subscribe-account-activity/migration/us-ss-migration-guide.html" rel="nofollow noreferrer">these migration instructions</a>, I've set up a webhook endpoint on my site, <a href="https://developer.twitter.com/en/docs/accounts-and-users/subscribe-account-activity/guides/securing-webhooks" rel="nofollow noreferrer">as described here</a>. I've checked that process is working, by making sure that when I open <code>https://example.com/webhook_endpoint?crc_token=foo</code> in my browser, I get a token in response.</p>
  
  <p>Now I'm trying and failing to register my webhook. I'm using the following code, and getting a 403 response. </p>
  
  <pre><code>from requests_oauthlib import OAuth1Session
  import urllib
  
  CONSUMER_KEY = 'my consumer key'
  CONSUMER_SECRET = 'my consumer secret'
  ACCESS_TOKEN = 'my access token'
  ACCESS_SECRET = 'my access secret'
  
  twitter = OAuth1Session(CONSUMER_KEY,
                          client_secret=CONSUMER_SECRET,
                          resource_owner_key=ACCESS_TOKEN,
                          resource_owner_secret=ACCESS_SECRET)
  webhook_endpoint = urllib.parse.quote_plus('https://example.com/webhook/')
  url = 'https://api.twitter.com/1.1/account_activity/all/env-beta/'
      'webhooks.json?url={}'.format(webhook_endpoint)
  r = twitter.post(url)
  </code></pre>
  
  <p>403 response content: <code>{"errors":[{"code":200,"message":"Forbidden."}]}</code></p>
  
  <p>I can successfully post a status using the same session object and </p>
  
  <pre><code>r = twitter.post('https://api.twitter.com/1.1/statuses/update.json?status=Test')
  </code></pre>
  
  <p>What am I doing wrong here?</p>