It seems from the ipaddress documentation that 'a.contains(b)' would appropriate.  But to avoid confusion with 'in', you could say a.covers(b).

However, 'in' is already used for <address> in <network>, and you can already get subnets out of a network (via 'subnets'), so it isn't completely crazy to consider the network a container of subnets (of varying sizes, depending on the arguments to subnet).