The following code causes an assertion failure in FutureObj_finalize() (in
Modules/_asynciomodule.c):

import asyncio
asyncio.Future()._log_traceback = True


Maybe we should allow Python code to only set it to False, and raise a
ValueError in case Python code tries to set it to True?

(PR 2050 made _log_traceback writable. Are there any usecases for setting it to
True from Python code?)