Add `daemon` kwarg to threading.Timer

The Timer class inherits the Thread class, and the daemon mechanic along with it.

There are two ways of setting the daemonness of a Thread : in the constructor, and through a property between the creation of the thread and its start. However, the daemon keyword-only argument taken by Thread is not taken by Timer. Here is an overview of what works and what doesn’t.

t = Thread(..., daemon=True)

t = Thread(...)
t.daemon = True

t = Timer(...)
t.daemon = True

t = Timer(..., daemon=True) # that one doesn't work

My proposition is simply to include the daemon=True keyword-only argument also to the Timer class, for more ease of use.

The use case is for the scheduled timer not to block termination of the program (same as a normal Thread), which is already possible as the property-setting syntax shows.