ipaddress: 2001:1::3/128 (RFC 9665) missing from IPv6 _private_networks_exceptions, is_global returns False

Bug report

ipaddress.ip_address('2001:1::3').is_global returns False, but 2001:1::3/128
is registered by IANA as globally reachable.

IANA registration

The IANA IPv6 Special-Purpose Address Registry lists, as of the 2024-04 update:

Registry: https://www.iana.org/assignments/iana-ipv6-special-registry/iana-ipv6-special-registry.xhtml

2001:1::3/128 was added by RFC 9665 (DNS-SD Service Registration Protocol) and
appears in the registry with Globally Reachable = True, exactly like its
neighbours 2001:1::1/128 and 2001:1::2/128.

The bug

_IPv6Constants._private_networks contains 2001::/23, which makes the whole
block non-global by default; the per-address carve-outs live in
_IPv6Constants._private_networks_exceptions. That list currently has
2001:1::1/128 and 2001:1::2/128 but not 2001:1::3/128, so 2001:1::3
is incorrectly treated as not globally reachable.

The ::1/::2 exceptions were added in GH-113179 (fixing #113171); the table
was simply never updated for the 2024-04 ::3 registration.

Reproduce

On current main:

>>> import ipaddress
>>> ipaddress.ip_address('2001:1::3').is_global
False        # expected: True
>>> ipaddress.ip_address('2001:1::3').is_private
True         # expected: False
>>> ipaddress.ip_address('2001:1::1').is_global   # sibling, correct
True
>>> ipaddress.ip_address('2001:1::2').is_global   # sibling, correct
True

Fix

Add IPv6Network('2001:1::3/128') to _IPv6Constants._private_networks_exceptions,
mirroring the existing ::1/::2 entries. PR to follow.

Linked PRs

• gh-151050

Review the ipaddress module's IPv6 constants, focusing on _IPv6Constants._private_networks_exceptions and the existing 2001:1::1/128 and ::2/128 entries. Verify the reported reproduction and sibling behavior; done means 2001:1::3 reports is_global=True and is_private=False, with the relevant tests passing.