docs: def fetch_access_token(self, oauth_request):
          """Processes an access_token request and returns the
          access token on success.
          """
          version = self._get_version(oauth_request)
          consumer = self._get_consumer(oauth_request)
          try:
              verifier = self._get_verifier(oauth_request)
          except Error:
              verifier = None
          # Get the request token.
          token = self._get_token(oauth_request, 'request')
          self._check_signature(oauth_request, consumer, token)
          new_token = self.data_store.fetch_access_token(consumer, token, verifier)
          return new_token