Make matplotlib import truly optional in image_tests.py
Make matplotlib import truly optional in image_tests.py

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