Use Python3 attr name for test exitfunc (#36)

Author: coretl

tests/sim_asyncio_ioc.py

@@ -39,5 +39,5 @@ async def callback(value):
     time.sleep(1)
     # Make sure coverage is written on epicsExit
     from pytest_cov.embed import cleanup
-    sys.exitfunc = cleanup
+    sys._run_exitfuncs = cleanup
     softioc.interactive_ioc()