Update oldest required `asdf` version in tox.ini
Update oldest required `asdf` version in tox.ini

deef87c9838ba63bcb847fcabad4c66d4556a151 updated the `asdf` version
requirement in `setup.cfg`, but not in `tox.ini`.

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