[3.12] gh-126451: Register contextvars.Context to collections.abc.Mapping (GH-126452) (#126519)

gh-126451: Register contextvars.Context to collections.abc.Mapping (GH-126452)
(cherry picked from commit 5dc36dc5658f6ba9cfd9d7a2771baaf17d2ee23a)

Co-authored-by: Stephen Morton <git@tungol.org>
Co-authored-by: sobolevn <mail@sobolevn.me>
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Co-authored-by: Peter Bierma <zintensitydev@gmail.com>