octoprint: add missing dependency to websocket-client

The `six` dependency was removed from the websocket-client package when
it was upgraded to 1.1.0 in 600d787b9b.
Since we pin a different version for octoprint, we should re-add it
here.

pkgs/applications/misc/octoprint/default.nix

@@ -44,6 +44,7 @@ let
                   inherit version;
                   sha256 = "63509b41d158ae5b7f67eb4ad20fecbb4eee99434e73e140354dc3ff8e09716f";
                 };
+                propagatedBuildInputs = [ self.six ];
               }
             );
           }