[3.14] gh-115119: Recommend upstream libmpdec in build requirements (GH-136205) (#136244)

gh-115119: Recommend upstream libmpdec in build requirements (GH-136205)
(cherry picked from commit b2e498ac26ba4720fa8a6a466aa8416ab69754d7)

Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>
Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>
Co-authored-by: Victor Stinner <vstinner@python.org>