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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:
 async def get_upstream_dns(cls) -> list: """Upstream DNS server addresses. Upstream DNS servers used to resolve domains not managed by this MAAS (space-separated IP addresses). Only used when MAAS is running its own DNS server. This value is used as the value of 'forwarders' in the DNS server config. """

        data = await cls.get_config("upstream_dns")
        return [] if data is None else re.split(r'[,\s]+', data)