[3.13] gh-124217, ipaddress: Add RFC 9637 reserved IPv6 block `3fff::/20` (GH-124240) (#124282)

gh-124217, ipaddress: Add RFC 9637 reserved IPv6 block `3fff::/20` (GH-124240)
(cherry picked from commit db6eb3640a7d98db6fea17cf9da4cb14504e5571)

Signed-off-by: y5c4l3 <y5c4l3@proton.me>
Co-authored-by: Y5 <124019959+y5c4l3@users.noreply.github.com>
3 files changed