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b0984a2868
## Summary
This PR corrects a misunderstanding I had related to Python's handling
of bound exceptions.
Previously, I thought this code ran without error:
```py
def f():
x = 1
try:
1 / 0
except Exception as x:
pass
print(x)
```
My understanding was that `except Exception as x` bound `x` within the
`except` block, but then restored the `x = 1` binding after exiting the
block.
In practice, however, this throws a `UnboundLocalError` error, because
`x` becomes "unbound" after exiting the exception handler. It's similar
to a `del` statement in this way.
This PR removes our behavior to "restore" the previous binding. This
could lead to faulty analysis in conditional blocks due to our lack of
control flow analysis, but those same problems already exist for `del`
statements.