Let DNSError inherit OSError to match with stdlib behavior

[Joongi Kim]

Both socket.getaddrinfo and asyncio.BaseEventLoop.getaddrinfo raises OSError when a DNS lookup fails.
It would be more consistent if aiodns.error.DNSError is also an instance of OSError, setting errno and stringified error message as the current implementation does.

[Saúl Ibarra Corretgé]

I don't think that's a good idea. The returned errors don't necessarily match errno.