[3.12] gh-123409: fix `IPv6Address.reverse_pointer` for IPv4-mapped addresses (GH-123419) (GH-135086)

Fix functionality that was broken with better textual representation for IPv4-mapped addresses (gh-87799)
(cherry picked from commit 77a2fb4bf1a1b160d6ce105508288fc77f636943)

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>