fix: try blocks delete context variable afterwards, but marimo still considers them refs or defs if used

[dmadisetti]

📝 Summary

previously

try:
   1 / 0
except Exception as e:
   f = e
   e = 2
refs(), defs()

Would give ref=e, def=e,f. At first glance, this seems fine- but "e" is scoped to the try block and is deleted afterwards, so the correct solution is ref=[], def=[f]

Note in the case

e = 7
try:
   1 / 0
except Exception as e:
   f = e
   e = 2
   print("woops")

e is no longer set if the except block triggers, but because it was set outside of the block, ref=, def=e,f is correct

🔍 Description of Changes

Added an ObscuredScope context to remove Name references if they are obscured by the try block + added some unit tests

Your PR will be reviewed more quickly if you can figure out the right person to tag with @ -->
@akshayka OR @mscolnick

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[mscolnick]

nice find and fix!