TST: Pin mpl for linkcheck.
TST: Pin mpl for linkcheck.

This only affects cron job. [ci skip]

Co-authored-by: Brigitta Sipocz <dcc912dd774217c9f555d59d5b344723f01d11f5@gmail.com>

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