Improve structure for specifying fixed format [skip ci]
Improve structure for specifying fixed format [skip ci]

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