require more recent sphinx

dwhswenson · dwhswenson · commit 2d71c69fd7ae · 2020-09-26T20:56:14.000+02:00
diff --git a/docs/requirements.txt b/docs/requirements.txt
@@ -3,3 +3,4 @@ numpy
 
 packaging
 sphinx-click
+sphinx >= 3.1  # will this work?
