Fix syntax error false positives on parenthesized context managers

[ntBre]

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:

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:

> 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 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.

[github-actions]

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