devel/mercurial: fix hg built against python314

Reported by: Gunther Nikl
PR: 292050
MFH: 2025Q4
(2026Q1 has not yet been created)
Approved by: portmgr@ (just-fix-it blanket)
Obtained from: https://foss.heptapod.net/mercurial/mercurial-devel/-/commit/fb8373b37f6e9942f7def675c9051b7ac5e727b4

(cherry picked from commit 5b7560e401d4ff4f5c7b5e3d81cc949b126572a1)