[syntax-errors] Parenthesized keyword argument names after Python 3.8 (#16482)

Summary
--

Unlike the other syntax errors detected so far, parenthesized keyword
arguments are only allowed *before* 3.8. It sounds like they were only
accidentally allowed before that [^1].

As an aside, you get a pretty confusing error from Python for this, so
it's nice that we can catch it:

```pycon
>>> def f(**kwargs): ...
... f((a)=1)
...
  File "<python-input-0>", line 2
    f((a)=1)
       ^^^
SyntaxError: expression cannot contain assignment, perhaps you meant "=="?
>>>
```
Test Plan
--
Inline tests.

[^1]: https://github.com/python/cpython/issues/78822

crates/ruff_python_parser/src/parser/mod.rs

@@ -441,7 +441,7 @@ impl<'src> Parser<'src> {
     /// Add an [`UnsupportedSyntaxError`] with the given [`UnsupportedSyntaxErrorKind`] and
     /// [`TextRange`] if its minimum version is less than [`Parser::target_version`].
     fn add_unsupported_syntax_error(&mut self, kind: UnsupportedSyntaxErrorKind, range: TextRange) {
-        if self.options.target_version < kind.minimum_version() {
+        if kind.is_unsupported(self.options.target_version) {
             self.unsupported_syntax_errors.push(UnsupportedSyntaxError {
                 kind,
                 range,