`ligo-skymap-plot-stats` does not work with matplotlib 3.0.0
ligo-skymap-plot-stats silently produces no output plots when run with matplotlib 3.0.0. Upgrading to matplotlib 3.0.3 fixes the problem. This is related to using an extension-less filename in
savefig and relying on rcParams for setting the format. Apparently this mechanism did not work on matplotlib 3.0.0 and it works in 3.0.3. Worth nudging the matplotlib required version to 3.0.3?