[SizzleUnrlsd]

Summary

try with exception edges, plus await and yield: the last open bullets of the PyHIR node set (#25) and of the CFG item (#11).

• Acceptance tests first (test: define try, exception edges, await and yield behavior), implementation follows.
• PyHIR: Try with ExceptHandler (type, bound name, body), else and finally; Await and Yield expressions. except* groups and yield from stay explicit diagnostics.
• CFG (§5 exception edges): every block laid out inside a try body carries exception edges to the handler blocks, so a raise or a call in the body can reach a handler; cfg.successors and predecessors include them, and so do dominance, reachability and back edges. Handlers, else and finally join after the statement; finally is modelled on the normal path only, exceptional exits are an approximation. Integrity checks cover exception targets.
• PyIR: blocks carry their exception targets (try_1 [except: handler_1]: in --emit-ir), handlers start with catch, and await / yield are instructions. SSA places phis at handlers and joins through the exception edges.
• Data-flow problems (constants, dependencies, taint) send a block's exit state along its exception edges, so taint reaches handlers and finally bodies.

--check no longer reports functions using try as unsupported.

Closes #25 (its last open bullets) and the exception-edges bullet of #11.