Fix isort config
Fix isort config

Fix #11496. `length_sort_stdlib` no more works with a recent version of
isort.

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