In order to calculate the number of octets to pad, we need to know the
size of the DNS query so far. We use dnspython to generate the wire
query and get its length. We add an EDNS(0) field so we have to add 4
octets which are the size of the Option code and the Option length. Then
we can calculate the number of octets to pad based on the Block-length
strategy (using 128 octets block size).
See RFC 7830 and 8467.