Fix syntax error false positives on parenthesized context managers

This PR resolves the issue noticed in
#20777 (comment). Namely,
cases like this were being flagged as syntax errors despite being perfectly
valid on Python 3.8:

```pycon
Python 3.8.20 (default, Oct  2 2024, 16:34:12)
[Clang 18.1.8 ] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> with (open("foo.txt", "w")): ...
...
Ellipsis
>>> with (open("foo.txt", "w")) as f: print(f)
...
<_io.TextIOWrapper name='foo.txt' mode='w' encoding='UTF-8'>
```

The second of these was already allowed but not the first:

```shell
> ruff check --target-version py38 --ignore ALL - <<EOF
with (open("foo.txt", "w")): ...
with (open("foo.txt", "w")) as f: print(f)
EOF
invalid-syntax: Cannot use parentheses within a `with` statement on Python 3.8 (syntax was added in Python 3.9)
 --> -:1:6
  |
1 | with (open("foo.txt", "w")): ...
  |      ^
2 | with (open("foo.txt", "w")) as f: print(f)
  |

Found 1 error.
```

There was some discussion of related cases in
#16523 (comment), but it
seems I overlooked the single-element case when examining tuple special cases.
This PR also removes the existing false positive for tuples that we
intentionally accepted there. If I'm understanding correctly, it turns out that
we didn't need to modify the parser, and cases like:

```python
with (a, b, c) as foo: ...  # parses but fails at runtime, tuple doesn't have __enter__
```

and

```

Author: ntBre