backward compat for private methods in registry
backward compat for private methods in registry

Signed-off-by: Nathaniel Starkman (@nstarman) <ac14c573519de521a9fc5a84607d51c0c956190b@protonmail.com>

StuartLittlefair/astropy,astropy/astropy,mhvk/astropy,astropy/astropy,astropy/astropy,lpsinger/astropy,mhvk/astropy,saimn/astropy,pllim/astropy,lpsinger/astropy,StuartLittlefair/astropy,larrybradley/astropy,lpsinger/astropy,mhvk/astropy,larrybradley/astropy,StuartLittlefair/astropy,pllim/astropy,saimn/astropy,astropy/astropy,larrybradley/astropy,StuartLittlefair/astropy,lpsinger/astropy,astropy/astropy,lpsinger/astropy,pllim/astropy,larrybradley/astropy,pllim/astropy,saimn/astropy,mhvk/astropy,saimn/astropy,StuartLittlefair/astropy,mhvk/astropy,larrybradley/astropy,pllim/astropy,saimn/astropy