gh-155140: Fix ipaddress.IPv6Address.is_link_local to require the strict fe80::/64 range

[Sudeep-A-Kulkarni]

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.

[bedevere-app]

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[picnixz]

@simitana Avoid low-quality review comments entirely generated by an LLM, it neither gives confidence in the comment nor does it actually help. It also restates unnecessary constraints that the OP can clearly see by looking at the CI.