[syntax-errors] Parenthesized context managers before Python 3.9 (#16523)

Summary
--

I thought this was very complicated based on the comment here:
https://github.com/astral-sh/ruff/pull/16106#issuecomment-2653505671 and
on some of the discussion in the CPython issue here:
https://github.com/python/cpython/issues/56991. However, after a little
bit of experimentation, I think it boils down to this example:

```python
with (x as y): ...
```

The issue is parentheses around a `with` item with an `optional_var`, as
we (and
[Python](https://docs.python.org/3/library/ast.html#ast.withitem)) call
the trailing variable name (`y` in this case). It's not actually about
line breaks after all, except that line breaks are allowed in
parenthesized expressions, which explains the validity of cases like


```pycon
>>> with (
...     x,
...     y
... ) as foo:
...     pass
... 
```

even on Python 3.8.

I followed [pyright]'s example again here on the diagnostic range (just
the opening paren) and the wording of the error.


Test Plan
--
Inline tests

[pyright]:
https://pyright-play.net/?pythonVersion=3.7&strict=true&code=FAdwlgLgFgBAFAewA4FMB2cBEAzBCB0EAHhJgJQwCGAzjLgmQFwz6tA

crates/ruff_python_parser/src/parser/statement.rs

@@ -2066,8 +2066,49 @@ impl<'src> Parser<'src> {
             return vec![];
         }
 
+        let open_paren_range = self.current_token_range();
+
         if self.at(TokenKind::Lpar) {
             if let Some(items) = self.try_parse_parenthesized_with_items() {
+                // test_ok tuple_context_manager_py38
+                // # parse_options: {"target-version": "3.8"}
+                // with (
+                //   foo,
+                //   bar,
+                //   baz,
+                // ) as tup: ...
+
+                // test_err tuple_context_manager_py38
+                // # parse_options: {"target-version": "3.8"}
+                // # these cases are _syntactically_ valid before Python 3.9 because the `with` item
+                // # is parsed as a tuple, but this will always cause a runtime error, so we flag it
+                // # anyway
+                // with (foo, bar): ...
+                // with (
+                //   open('foo.txt')) as foo: ...
+                // with (
+                //   foo,
+                //   bar,
+                //   baz,
+                // ): ...
+                // with (foo,): ...
+
+                // test_ok parenthesized_context_manager_py39
+                // # parse_options: {"target-version": "3.9"}
+                // with (foo as x, bar as y): ...
+                // with (foo, bar as y): ...
+                // with (foo as x, bar): ...
+
+                // test_err parenthesized_context_manager_py38
+                // # parse_options: {"target-version": "3.8"}
+                // with (foo as x, bar as y): ...
+                // with (foo, bar as y): ...
+                // with (foo as x, bar): ...
+                self.add_unsupported_syntax_error(
+                    UnsupportedSyntaxErrorKind::ParenthesizedContextManager,
+                    open_paren_range,
+                );
+
                 self.expect(TokenKind::Rpar);
                 items
             } else {