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