Ignore warning from astropy.io.votable for <v4.1

Author: bsipocz

setup.cfg

@@ -56,6 +56,8 @@ filterwarnings =
 # Upstream, remove when fixed, PRs have been opened
     ignore::DeprecationWarning:pyvo
     ignore::DeprecationWarning:regions
+# This should be cleared once we requre astropy>=4.1
+    ignore:tostring\(\) is deprecated. Use tobytes:DeprecationWarning:astropy
 markers =
     bigdata: marks tests that are expected to trigger a large download (deselect with '-m "not bigdata"')