Factor out common add/sub for TimeDelta
Factor out common add/sub for TimeDelta

larrybradley/astropy,aleksandr-bakanov/astropy,dhomeier/astropy,mhvk/astropy,aleksandr-bakanov/astropy,lpsinger/astropy,mhvk/astropy,astropy/astropy,pllim/astropy,StuartLittlefair/astropy,larrybradley/astropy,larrybradley/astropy,dhomeier/astropy,StuartLittlefair/astropy,aleksandr-bakanov/astropy,StuartLittlefair/astropy,mhvk/astropy,astropy/astropy,pllim/astropy,dhomeier/astropy,lpsinger/astropy,lpsinger/astropy,saimn/astropy,saimn/astropy,larrybradley/astropy,saimn/astropy,astropy/astropy,saimn/astropy,lpsinger/astropy,dhomeier/astropy,saimn/astropy,larrybradley/astropy,astropy/astropy,pllim/astropy,mhvk/astropy,pllim/astropy,pllim/astropy,StuartLittlefair/astropy,mhvk/astropy,dhomeier/astropy,astropy/astropy,lpsinger/astropy,StuartLittlefair/astropy,aleksandr-bakanov/astropy