Treat exception binding as explicit deletion (#5057)

## 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.

crates/ruff/src/checkers/ast/mod.rs

@@ -3944,7 +3944,6 @@ where
                             );
                         }
 
-                        let definition = self.semantic_model.scope().get(name);
                         self.handle_node_store(
                             name,
                             &Expr::Name(ast::ExprName {
@@ -3979,11 +3978,6 @@ where
                                 }
                             }
                         }
-
-                        if let Some(binding_id) = definition {
-                            let scope = self.semantic_model.scope_mut();
-                            scope.add(name, binding_id);
-                        }
                     }
                     None => walk_excepthandler(self, excepthandler),
                 }

crates/ruff/src/rules/pyflakes/mod.rs

@@ -311,6 +311,20 @@ mod tests {
     "#,
         "del_shadowed_local_import_in_local_scope"
     )]
+    #[test_case(
+        r#"
+        def f():
+            x = 1
+
+            try:
+                1 / 0
+            except Exception as x:
+                pass
+
+            print(x)
+            "#,
+        "print_after_shadowing_except"
+    )]
     fn contents(contents: &str, snapshot: &str) {
         let diagnostics = test_snippet(contents, &Settings::for_rules(&Linter::Pyflakes));
         assert_messages!(snapshot, diagnostics);

crates/ruff/src/rules/pyflakes/snapshots/ruff__rules__pyflakes__tests__print_after_shadowing_except.snap

@@ -0,0 +1,32 @@
+---
+source: crates/ruff/src/rules/pyflakes/mod.rs
+---
+<filename>:7:25: F841 [*] Local variable `x` is assigned to but never used
+  |
+5 |     try:
+6 |         1 / 0
+7 |     except Exception as x:
+  |                         ^ F841
+8 |         pass
+  |
+  = help: Remove assignment to unused variable `x`
+
+ℹ Suggested fix
+4 4 | 
+5 5 |     try:
+6 6 |         1 / 0
+7   |-    except Exception as x:
+  7 |+    except Exception:
+8 8 |         pass
+9 9 | 
+10 10 |     print(x)
+
+<filename>:10:11: F821 Undefined name `x`
+   |
+ 8 |         pass
+ 9 | 
+10 |     print(x)
+   |           ^ F821
+   |
+
+