[3.12] gh-131325: fix sendfile fallback implementation to drain data after writing to transport (GH-131376) (#131378)

gh-131325: fix sendfile fallback implementation to drain data after writing to transport (GH-131376)
(cherry picked from commit 94f4d87aeb4d2d7bddcb4c3aad4f62a727ac91ee)

Co-authored-by: Kumar Aditya <kumaraditya@python.org>