procutil: make recent fix for zombies compatible with py2

The fix in ed684a82e29b (procutil: always waiting on child processes
to prevent zombies with 'hg serve', 2020-05-07) works only on Python 3
because it passes a `daemon` argument to `threading.Thread()`. Python
2 requires you to assign to the `.daemon` property instead. Python 3
also seems to support that, so this patch fixes the code by
unconditionally using the old form.

Differential Revision: https://phab.mercurial-scm.org/D8657

mercurial/utils/procutil.py

@@ -615,7 +615,9 b' else:'
                     def _do_wait():
                         os.waitpid(pid, 0)
 
-                    threading.Thread(target=_do_wait, daemon=True).start()
+                    t = threading.Thread(target=_do_wait)
+                    t.daemon = True
+                    t.start()
                     return
                 # Parent process
                 (_pid, status) = os.waitpid(pid, 0)