[3.13] gh-120974: Make _asyncio._enter_task atomic in the free-threaded build (GH-122138) (#122152)

gh-120974: Make _asyncio._enter_task atomic in the free-threaded build (GH-122138)

Use `PyDict_SetDefaultRef` to set the current task in a single operation
under the dictionary's lock.
(cherry picked from commit 47847aa8ef66837f984fc4e30187d88f8d8ab201)

Co-authored-by: Sam Gross <colesbury@gmail.com>