dnspython accepts a port as part of the nameserver.
Currently, the nameservers are passed as strings which
leads dnspython to create Nameserver objects out of them
using the port that is currently set in the Resolver instance.
That creation of Nameserver objects is done right when the
`nameservers` property is set.
If a port is to be set by us, the `port` attribute of the
Resolver needs to be set before the nameservers are passed
to the Resolver so when the nameservers are passed, that new
port is used to create the Nameserver objects.
Therefore, the assignment of the `nameservers` property of the
Resolver is moved after the argument processing so the `port`
attribute is (if it's given in the lookup-call) definitely set
before the `nameservers` property.
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