added rfc 7871 parsing: client subnet in dns queries

[szengerle]

edn0 entry parser added; returns EDNSOptionECS object; ipv4 and v6 support;

[paulc]

Hi,

Thanks for providing the code, this looks useful to add as a feature and I'd be happy to merge with a couple of minor changes:

• I looks like it would be better if EDNSOptionECS was a subclass of EDNSOption and supported the same interface (including pack/unpack and the other methods)
• It would also be good if you could include some doctests
• Unfortunately I don't have access to any client subnet test data so it would be good to add some to dnslib/test so that this is exercised by run_tests.sh - it is possible to generate these from a server response using python3 -m dnslib.test_decode --new however given that the client subnet would only be visible at the server side you might need to manually create the test file in the dnslib/test dir (you just need a hexdump of the query/response).
• I want to retain Python2 compatibility so please don't remove python from run_tests.sh, if you only want to test with a specific version just use the VERSIONS environment variable (VERSIONS=python3 ./run_tests.sh)
• It would be easier if your diff could ignore whitespace changes - I know that this needs to be cleaned up and just committed a change to clean up on master but it makes it harder to work out the important changes in the diff.

Thanks, Paul

[paulc]

I'd still like to implement this but need to update PR as above - closing for now