tracemalloc no longer acquires the GIL nor creates a temporary thread
state when tracing memory allocations. A thread with no attached thread
state now records the trace with the "<unknown>" traceback instead of
attaching a thread state to capture the Python traceback. Threads
without a thread state used to pay for a GIL acquisition plus a full
thread state creation and destruction on every traced raw allocation,
only to record an empty traceback anyway.