v1.2.3

get_start_control_flow_node, next_from_end, raise edges, and labels in branches (#6)

* Adds get_start_control_flow_node to ControlFlowGraph
* Adds next_from_end to ControlFlowNode
* Uses labels (e.g. '<exit>' and '<raise>' strings) to indicate these special nodes
* Support keyword only arguments without defaults
* Add non-interrupting edges from raise statements
* Bump version number