Fix data file creation in pytest fixture...
Fix data file creation in pytest fixture...

Implicitly treating the pytest `tmpdir` fixture as a string does not
work in all versions of Python.

aleksandr-bakanov/astropy,DougBurke/astropy,MSeifert04/astropy,bsipocz/astropy,pllim/astropy,StuartLittlefair/astropy,StuartLittlefair/astropy,astropy/astropy,funbaker/astropy,bsipocz/astropy,aleksandr-bakanov/astropy,saimn/astropy,StuartLittlefair/astropy,mhvk/astropy,StuartLittlefair/astropy,pllim/astropy,larrybradley/astropy,mhvk/astropy,aleksandr-bakanov/astropy,astropy/astropy,astropy/astropy,dhomeier/astropy,larrybradley/astropy,pllim/astropy,lpsinger/astropy,MSeifert04/astropy,larrybradley/astropy,lpsinger/astropy,saimn/astropy,dhomeier/astropy,saimn/astropy,saimn/astropy,astropy/astropy,DougBurke/astropy,astropy/astropy,mhvk/astropy,lpsinger/astropy,funbaker/astropy,DougBurke/astropy,bsipocz/astropy,funbaker/astropy,mhvk/astropy,MSeifert04/astropy,larrybradley/astropy,aleksandr-bakanov/astropy,pllim/astropy,stargaser/astropy,stargaser/astropy,StuartLittlefair/astropy,bsipocz/astropy,lpsinger/astropy,dhomeier/astropy,dhomeier/astropy,pllim/astropy,funbaker/astropy,mhvk/astropy,MSeifert04/astropy,larrybradley/astropy,stargaser/astropy,dhomeier/astropy,saimn/astropy,stargaser/astropy,DougBurke/astropy,lpsinger/astropy