Add the 5f00::/16 (RFC 9602) and 100:0:0:1::/64 (RFC 9780) special-purpose IPv6 ranges to ipaddress's _private_networks so is_global/is_private match the IANA registry.

Fixes #151749

Problem

Both ranges are listed in the IANA IPv6 Special-Purpose Address Registry with Globally Reachable = False, but ipaddress currently reports them as globally reachable:

• 5f00::/16 — SRv6 SIDs, defined in RFC 9602 (Globally Reachable: False).
• 100:0:0:1::/64 — Dummy IPv6 Prefix, defined in RFC 9780 (Globally Reachable: False). Note this block is disjoint from the already-listed 100::/64 (RFC 6666 Discard-Only), so it is not currently covered.

Before this change IPv6Address.is_global returns True (and is_private returns False) for addresses in both blocks, which is incorrect.

Fix

Adds the two networks to _IPv6Constants._private_networks, alongside the existing not-globally-reachable IPv6 entries. This follows the precedent set by CVE-2024-4032 / gh-113171 and gh-124217 (3fff::/20, RFC 9637), which added sibling ranges to these same tables.

Scope: This PR is intentionally limited to _private_networks. It does not touch _private_networks_exceptions; the related 2001:1::3/128 (RFC 9665) exception is handled separately in #151050 and is deliberately left unchanged here.

Behaviour (stock vs. patched)

Addresses immediately adjacent to each new block are unchanged. A regression test covering both the changed and adjacent addresses is added to testReservedIpv6, and a Misc/NEWS.d entry is included.

• Issue: ipaddress: Two more IPv6 ranges (RFC 9602, 9780) missing from _private_networks #151749