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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def remember(self, request, username, **kw):
 """ Returns 'WWW-Authenticate' header with a value that should be used in 'Authorization' header. """

        if self.credentials_callback:
            token = self.credentials_callback(username, request)
            api_key = 'ApiKey {}:{}'.format(username, token)
            return [('WWW-Authenticate', api_key)]