Fix YTT201 for '!=' comparisons (#18293)

## Summary

Closes #18292.

Author: charliermarsh

crates/ruff_linter/src/rules/flake8_2020/rules/compare.rs

@@ -52,17 +52,20 @@ impl Violation for SysVersionCmpStr3 {
 
 /// ## What it does
 /// Checks for equality comparisons against the major version returned by
-/// `sys.version_info` (e.g., `sys.version_info[0] == 3`).
+/// `sys.version_info` (e.g., `sys.version_info[0] == 3` or `sys.version_info[0] != 3`).
 ///
 /// ## Why is this bad?
 /// Using `sys.version_info[0] == 3` to verify that the major version is
 /// Python 3 or greater will fail if the major version number is ever
 /// incremented (e.g., to Python 4). This is likely unintended, as code
 /// that uses this comparison is likely intended to be run on Python 2,
-/// but would now run on Python 4 too.
+/// but would now run on Python 4 too. Similarly, using `sys.version_info[0] != 3`
+/// to check for Python 2 will also fail if the major version number is
+/// incremented.
 ///
 /// Instead, use `>=` to check if the major version number is 3 or greater,
-/// to future-proof the code.
+/// or `<` to check if the major version number is less than 3, to future-proof
+/// the code.
 ///
 /// ## Example
 /// ```python
@@ -88,12 +91,18 @@ impl Violation for SysVersionCmpStr3 {
 /// - [Python documentation: `sys.version`](https://docs.python.org/3/library/sys.html#sys.version)
 /// - [Python documentation: `sys.version_info`](https://docs.python.org/3/library/sys.html#sys.version_info)
 #[derive(ViolationMetadata)]
-pub(crate) struct SysVersionInfo0Eq3;
+pub(crate) struct SysVersionInfo0Eq3 {
+    eq: bool,
+}
 
 impl Violation for SysVersionInfo0Eq3 {
     #[derive_message_formats]
     fn message(&self) -> String {
-        "`sys.version_info[0] == 3` referenced (python4), use `>=`".to_string()
+        if self.eq {
+            "`sys.version_info[0] == 3` referenced (python4), use `>=`".to_string()
+        } else {
+            "`sys.version_info[0] != 3` referenced (python4), use `<`".to_string()
+        }
     }
 }
 
@@ -235,7 +244,7 @@ pub(crate) fn compare(checker: &Checker, left: &Expr, ops: &[CmpOp], comparators
             {
                 if *i == 0 {
                     if let (
-                        [CmpOp::Eq | CmpOp::NotEq],
+                        [operator @ (CmpOp::Eq | CmpOp::NotEq)],
                         [
                             Expr::NumberLiteral(ast::ExprNumberLiteral {
                                 value: ast::Number::Int(n),
@@ -246,7 +255,9 @@ pub(crate) fn compare(checker: &Checker, left: &Expr, ops: &[CmpOp], comparators
                     {
                         if *n == 3 && checker.enabled(Rule::SysVersionInfo0Eq3) {
                             checker.report_diagnostic(Diagnostic::new(
-                                SysVersionInfo0Eq3,
+                                SysVersionInfo0Eq3 {
+                                    eq: matches!(*operator, CmpOp::Eq),
+                                },
                                 left.range(),
                             ));
                         }

crates/ruff_linter/src/rules/flake8_2020/snapshots/ruff_linter__rules__flake8_2020__tests__YTT201_YTT201.py.snap

@@ -20,7 +20,7 @@ YTT201.py:8:7: YTT201 `sys.version_info[0] == 3` referenced (python4), use `>=`
 10 | PY2 = version_info[0] != 3
    |
 
-YTT201.py:9:7: YTT201 `sys.version_info[0] == 3` referenced (python4), use `>=`
+YTT201.py:9:7: YTT201 `sys.version_info[0] != 3` referenced (python4), use `<`
    |
  7 | PY3 = sys.version_info[0] == 3
  8 | PY3 = version_info[0] == 3
@@ -29,7 +29,7 @@ YTT201.py:9:7: YTT201 `sys.version_info[0] == 3` referenced (python4), use `>=`
 10 | PY2 = version_info[0] != 3
    |
 
-YTT201.py:10:7: YTT201 `sys.version_info[0] == 3` referenced (python4), use `>=`
+YTT201.py:10:7: YTT201 `sys.version_info[0] != 3` referenced (python4), use `<`
    |
  8 | PY3 = version_info[0] == 3
  9 | PY2 = sys.version_info[0] != 3