DOC: Need matplotlib>=3.1 to build
DOC: Need matplotlib>=3.1 to build

Co-authored-by: Erik M. Bray <13694847bdacb3ad5c2e2411f328f0f2a0a8420d@gmail.com>

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