decode scoped IPv6 addresses instead of rejecting them

[sahvx655-wq]

Changes

A scoped IPv6 address survives encoding but blows up on the way back in. encode_ipv6_address serialises an IPv6Address that carries a scope_id as the tag 54 array [address, null, zone id], but decode_ipv6 only recognised the [prefix, address] network form and the [address, prefix] interface form. The null second element falls straight through to the catch-all, so loads(dumps(IPv6Address("fe80::1%eth0"))) raises CBORDecodeError: invalid types in input array rather than returning the address. Anything holding a link-local address with a zone id therefore cannot be round-tripped, and the failure is silent until you actually feed such a value back through the decoder.

I hoisted the zone id suffix formatting above the type match (it is common to every array form) and added a branch that reads a null second element with a bytes first element as a bare scoped address, constructing the IPv6Address with the %zone suffix directly instead of routing it through the network/interface class. The network and interface paths, including the over-length address check, are untouched.

Checklist

• You've added tests (in tests/) which would fail without your patch
• You've updated the documentation (in docs/), in case of behavior changes or new features
• You've added a new changelog entry (in docs/versionhistory.rst).

[coveralls]

coverage: 94.682% (+0.008%) from 94.674% — sahvx655-wq:ipv6-scoped-address-decode into agronholm:master