[3.12] gh-128078: Clear exception in `anext` before calling `_PyGen_SetStopIterationValue` (GH-128780) (#128784)

gh-128078: Clear exception in `anext` before calling `_PyGen_SetStopIterationValue` (GH-128780)
(cherry picked from commit 76ffaef729c91bb79da6df2ade48f3ec51118300)

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
Co-authored-by: Kumar Aditya <kumaraditya@python.org>