code: def authenticate(self, req, resp, resource):
          
          username, password = self._extract_credentials(req)
          user = self.user_loader(username, password)
          if not user:
              raise falcon.HTTPUnauthorized(
                  description='Invalid Username/Password')
  
          return user
docs: Extract basic auth token from request `authorization` header,  deocode the
          token, verifies the username/password and return either a ``user``
          object if successful else raise an `falcon.HTTPUnauthoried exception`