Fix syntax error false positives on parenthesized context managers (#20846)

This PR resolves the issue noticed in
https://github.com/astral-sh/ruff/pull/20777#discussion_r2417233227.
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
https://github.com/astral-sh/ruff/pull/16523#discussion_r1984657793, but
it seems I overlooked the single-element case when flagging tuples. As
suggested in the other thread, we can just check if there's more than
one element or a trailing comma, which will cause the tuple parsing on
<=3.8 and avoid the false positives.

crates/ruff_python_parser/resources/inline/err/tuple_context_manager_py38.py

@@ -3,8 +3,6 @@
 # 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,

crates/ruff_python_parser/resources/inline/ok/single_parenthesized_item_context_manager_py38.py

@@ -0,0 +1,5 @@
+# parse_options: {"target-version": "3.8"}
+with (
+  open('foo.txt')) as foo: ...
+with (
+  open('foo.txt')): ...