[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/resources/inline/err/parenthesized_kwarg_py38.py

@@ -0,0 +1,4 @@
+# parse_options: {"target-version": "3.8"}
+f((a)=1)
+f((a) = 1)
+f( ( a ) = 1)

crates/ruff_python_parser/resources/inline/ok/parenthesized_kwarg_py37.py

@@ -0,0 +1,2 @@
+# parse_options: {"target-version": "3.7"}
+f((a)=1)