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.

…e attached

Store traceback frame filenames as interned NUL terminated UTF-8 strings
instead of Python str objects, so that capturing a traceback no longer
uses or modifies Python objects. Threads without an attached thread state
now capture their Python traceback by walking the frames of the thread
state most recently bound to the thread; only threads which never had a
thread state record the traceback as "<unknown>".