GeezerVison

Fixes gh-155140.

Bug description:

ipaddress.IPv6Address.is_link_local currently checks membership in the broad fe80::/10 block, but per RFC 4291 section 2.5.6 a true link-local address must have the first 64 bits equal to fe80:: exactly (10-bit prefix followed by 54 zero bits). This means addresses that fall inside fe80::/10 but outside fe80::/64 are incorrectly reported as link-local.

Reproducer:

import ipaddress
ipaddress.IPv6Address("fe80:6666:6666:6666:6666:6666:6666:6666").is_link_local

Current output on main: True
Expected output: False

Fix:

Change _IPv6Constants._linklocal_network from fe80::/10 to fe80::/64, matching the strict RFC 4291 definition.

Testing:

Updated the existing testReservedIpv6 assertions for fea0:: and febf:ffff:: (previously expected True, now correctly False since they fall outside fe80::/64), and added regression tests for fe80:: (still True) and the originally reported fe80:6666:... address (now False), for both ipaddress.ip_network and ipaddress.ip_address.