docs: def _GetAccessToken(self):
      """Gets oauth2 access token for Gitkit API using service account.
  
      Returns:
        string, oauth2 access token.
      """
      d = {
          'assertion': self._GenerateAssertion(),
          'grant_type': 'urn:ietf:params:oauth:grant-type:jwt-bearer',
      }
      try:
          body = parse.urlencode(d)
      except AttributeError:
          body = urllib.urlencode(d)
      req = urllib_request.Request(RpcHelper.TOKEN_ENDPOINT)
      req.add_header('Content-type', 'application/x-www-form-urlencoded')
      binary_body = body.encode('utf-8')
      raw_response = urllib_request.urlopen(req, binary_body)
      return simplejson.loads(raw_response.read())['access_token']