BUG: Bug in Timestamp-Timestamp not returning a Timedelta type (GH8865)
closes #8865 

cc @shoyer

was a rabbit-hole!

not 100% convinced that the auto-broadcasting strategy will work here, but go for it.
 generally looks pretty good to me!
 @shoyer great!

still would love to have you refactor when you have to time to use more array broadcasting stuff.
