Pin mpl as test dependency to avoid it pulling in newer numpy versions

Author: bsipocz

tox.ini

@@ -31,10 +31,12 @@ deps =
 
 # TODO: Add more versions to oldestdeps. numpy<1.15 could not be installed
 # in CI, a much newer version was pulled in instead, thus setting
-# minimum numpy to 1.15.
+# minimum numpy to 1.15. mpl while not a dependency, it's required for the
+# tests, and would pull up a newer numpy version if not pinned.
 
     oldestdeps: astropy==3.1.2
     oldestdeps: numpy==1.15
+    oldestdeps: matplotlib==3.3.*
     cov: codecov
 
 extras =