Closes #4264: Do not allocate subnet router anycast for IPv6 prefixes

[sc68cal]

This fixes #4264 by removing the first IPv6 address from the available_ips that is returned, in cases where the RFCs prohibit it

[sc68cal]

This just needs an additional test where there are some child prefixes, and then the cover message for this PR and the commit message itself needs cleanup, then this is good to go.

I'll also clean up the formatting of the test to make it more readable

[sc68cal]

We might want to reserve the anycast addresses defined in https://www.rfc-editor.org/rfc/rfc2526, if it's straight forward

EDIT: I might just make it a separate PR

[sc68cal]

I also see the following note in netaddr

https://netaddr.readthedocs.io/en/latest/api.html#netaddr.IPNetwork.iter_hosts

The next release (0.9.0) will contain a backwards incompatible change connected to handling of RFC 6164 IPv6 addresses (/127 and /128 subnets). When iterating IPNetwork and IPNetwork.iter_hosts() the first addresses in the networks will no longer be excluded.

We are currently using 0.8.0 - and this is probably something we're going to run into trouble with?

[jeremystretch]

Thanks for working on this!

We are currently using 0.8.0 - and this is probably something we're going to run into trouble with?

I don't think we use iter_hosts() anywhere but I'll keep an eye on it.