use Netlist class for circuit

[pepijndevos]

In init of this class circuit is set to a Netlist class that is a thin wrapper over a list. But then later it gets overwritten with an actual list. In practice it doesn't really matter since the .circuit accessor just returns an iterator over that internal list anyway, but it's just a correctness fix.

Alternatively, you could also change init to just use a list and nobody would notice.