[red-knot] Support for noteq None narrowing

crates/red_knot_python_semantic/resources/mdtest/conditionals.md

@@ -0,0 +1,15 @@
+# Conditionals
+
+## Narrowing
+
+### `is != None`
+
+```py
+x = None if flag else 1
+y = 0
+if x != None:
+    y = x
+
+reveal_type(x)  # revealed: None | Literal[1]
+reveal_type(y)  # revealed: Literal[0, 1]
+```

crates/red_knot_python_semantic/src/types/narrow.rs

@@ -155,7 +155,7 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
             let inference = infer_expression_types(self.db, expression);
             for (op, comparator) in std::iter::zip(&**ops, &**comparators) {
                 let comp_ty = inference.expression_ty(comparator.scoped_ast_id(self.db, scope));
-                if matches!(op, ast::CmpOp::IsNot) {
+                if matches!(op, ast::CmpOp::IsNot | ast::CmpOp::NotEq) {
                     let ty = IntersectionBuilder::new(self.db)
                         .add_negative(comp_ty)
                         .build();