Fix _tracemalloc start/stop

Issue #26588: Fix _tracemalloc start/stop: don't play with the reentrant flag.

set_reentrant(1) fails with an assertion error if tracemalloc_init() is called
first in a thread A and tracemalloc_start() is called second in a thread B. The
tracemalloc is imported in a thread A. Importing the module calls
tracemalloc_init(). tracemalloc.start() is called in a thread B.