Consider with blocks as single-item branches (#12311)

## Summary

Ensures that, e.g., the following is not considered a
redefinition-without-use:

```python
import contextlib

foo = None
with contextlib.suppress(ImportError):
    from some_module import foo
```

Closes https://github.com/astral-sh/ruff/issues/12309.

crates/ruff_linter/resources/test/fixtures/pyflakes/F811_31.py

@@ -0,0 +1,21 @@
+"""Regression test for: https://github.com/astral-sh/ruff/issues/12309"""
+
+import contextlib
+
+foo = None
+with contextlib.suppress(ImportError):
+    from some_module import foo
+
+bar = None
+try:
+    from some_module import bar
+except ImportError:
+    pass
+
+
+try:
+    baz = None
+
+    from some_module import baz
+except ImportError:
+    pass

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

@@ -928,6 +928,19 @@ impl<'a> Visitor<'a> for Checker<'a> {
                     self.visit_expr(expr);
                 }
             }
+            Stmt::With(ast::StmtWith {
+                items,
+                body,
+                is_async: _,
+                range: _,
+            }) => {
+                for item in items {
+                    self.visit_with_item(item);
+                }
+                self.semantic.push_branch();
+                self.visit_body(body);
+                self.semantic.pop_branch();
+            }
             Stmt::While(ast::StmtWhile {
                 test,
                 body,

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

@@ -126,6 +126,7 @@ mod tests {
     #[test_case(Rule::RedefinedWhileUnused, Path::new("F811_28.py"))]
     #[test_case(Rule::RedefinedWhileUnused, Path::new("F811_29.pyi"))]
     #[test_case(Rule::RedefinedWhileUnused, Path::new("F811_30.py"))]
+    #[test_case(Rule::RedefinedWhileUnused, Path::new("F811_31.py"))]
     #[test_case(Rule::UndefinedName, Path::new("F821_0.py"))]
     #[test_case(Rule::UndefinedName, Path::new("F821_1.py"))]
     #[test_case(Rule::UndefinedName, Path::new("F821_2.py"))]

crates/ruff_linter/src/rules/pyflakes/snapshots/ruff_linter__rules__pyflakes__tests__F811_F811_31.py.snap

@@ -0,0 +1,13 @@
+---
+source: crates/ruff_linter/src/rules/pyflakes/mod.rs
+---
+F811_31.py:19:29: F811 Redefinition of unused `baz` from line 17
+   |
+17 |     baz = None
+18 | 
+19 |     from some_module import baz
+   |                             ^^^ F811
+20 | except ImportError:
+21 |     pass
+   |
+   = help: Remove definition: `baz`