[3.12] gh-121390: tracemalloc: Fix tracebacks memory leak (GH-121391) (#121393)

gh-121390: tracemalloc: Fix tracebacks memory leak (GH-121391)

The tracemalloc_tracebacks hash table has traceback keys and NULL
values, but its destructors do not reflect this -- key_destroy_func is
NULL while value_destroy_func is raw_free. Swap these to free the
traceback keys instead.
(cherry picked from commit db39bc42f90c151b298f97b780e62703adbf1221)

Co-authored-by: Josh Brobst <jbrobst@proton.me>