prevent recursion error and enable physicaltype objects to be pickled
prevent recursion error and enable physicaltype objects to be pickled

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