class _IPv6Constants._reserved_networks should include fec0::/10

Bug report

This is a follow-up of: https://github.com/python/cpython/pull/136794

Bug description:

IPv6 address block fec0::/10 has been deprecated in 2004 as a "Site-Local scoped address prefix" and is now a `` (see Internet Protocol Version 6 Address Space and RFC 3879).

class _IPv6Constants._reserved_networks is still missing address block fec0::/10 from the list: https://github.com/python/cpython/blob/main/Lib/ipaddress.py#L2402-L2411

We should add that address block to the list and update the documentation (https://docs.python.org/3/library/ipaddress.html#ipaddress.IPv4Address.is_reserved)

The question is do we also:

• remove the is_site_local property since according to RFC 3879, these addresses have been completely deprecated? (= breaking change)
• keep the is_site_local property to avoid breaking changes, but always return False as there is no such address block anymore?

CPython versions tested on:

CPython main branch

Operating systems tested on:

Windows

Linked PRs

• gh-151927

从 Lib/ipaddress.py 中的类 _IPv6Constants._reserved_networks 开始，查看链接的后续 PR gh-151927，然后检查 issue 中引用的 ipaddress 文档部分。完成的标准是：fec0::/10 的行为和文档已得到一致更新，并且 is_site_local 的兼容性问题已解决。