Turns out print() does not flush I/O by default

and sys.stderr is not unbuffered except in interactive mode.  This is important here
a) To ensure that the 'Waiting...' message is displayed before waiting on flock() and
b) To ensure the last message is displayed before exec()-ing.

src/bin/sage-flock

@@ -85,9 +85,9 @@ def main(argv=None):
 
         print("Waiting for {0} lock to run {1} ... ".format(
             kind, ' '.join(pipes.quote(arg) for arg in command)), end='',
-            file=sys.stderr)
+            file=sys.stderr, flush=True)
         fcntl.flock(lock, locktype)
-        print("ok", file=sys.stderr)
+        print("ok", file=sys.stderr, flush=True)
 
     os.execvp(command[0], command)