get_start_control_flow_node, next_from_end, raise edges, and labels in branches

[dbieber]

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