[syntax-errors] Assignment expressions before Python 3.8 (#16383)

## Summary
This PR is the first in a series derived from
https://github.com/astral-sh/ruff/pull/16308, each of which add support
for detecting one version-related syntax error from
https://github.com/astral-sh/ruff/issues/6591. This one should be
the largest because it also includes the addition of the 
`Parser::add_unsupported_syntax_error` method

Otherwise I think the general structure will be the same for each syntax
error:
* Detecting the error in the parser
* Inline parser tests for the new error
* New ruff CLI tests for the new error

## Test Plan
As noted above, there are new inline parser tests, as well as new ruff
CLI
tests. Once https://github.com/astral-sh/ruff/pull/16379 is resolved,
there should also be new mdtests for red-knot,
but this PR does not currently include those.

crates/ruff_python_parser/src/parser/expression.rs

@@ -7,7 +7,7 @@ use rustc_hash::{FxBuildHasher, FxHashSet};
 use ruff_python_ast::name::Name;
 use ruff_python_ast::{
     self as ast, BoolOp, CmpOp, ConversionFlag, Expr, ExprContext, FStringElement, FStringElements,
-    IpyEscapeKind, Number, Operator, StringFlags, UnaryOp,
+    IpyEscapeKind, Number, Operator, PythonVersion, StringFlags, UnaryOp,
 };
 use ruff_text_size::{Ranged, TextLen, TextRange, TextSize};
 
@@ -16,7 +16,7 @@ use crate::parser::{helpers, FunctionKind, Parser};
 use crate::string::{parse_fstring_literal_element, parse_string_literal, StringType};
 use crate::token::{TokenKind, TokenValue};
 use crate::token_set::TokenSet;
-use crate::{FStringErrorType, Mode, ParseErrorType};
+use crate::{FStringErrorType, Mode, ParseErrorType, UnsupportedSyntaxErrorKind};
 
 use super::{FStringElementsKind, Parenthesized, RecoveryContextKind};
 
@@ -2161,10 +2161,24 @@ impl<'src> Parser<'src> {
 
         let value = self.parse_conditional_expression_or_higher();
 
+        let range = self.node_range(start);
+
+        // test_err walrus_py37
+        // # parse_options: { "target-version": "3.7" }
+        // (x := 1)
+
+        // test_ok walrus_py38
+        // # parse_options: { "target-version": "3.8" }
+        // (x := 1)
+
+        if self.options.target_version < PythonVersion::PY38 {
+            self.add_unsupported_syntax_error(UnsupportedSyntaxErrorKind::Walrus, range);
+        }
+
         ast::ExprNamed {
             target: Box::new(target),
             value: Box::new(value.expr),
-            range: self.node_range(start),
+            range,
         }
     }