Disallow assignment to '__class__' in TypeChecker.visit_assignment_stmt

Added a check inside TypeChecker.visit_assignment_stmt to disallow assignments to '__class__', as such assignments are unsafe and may lead to unpredictable behavior.

The new check raises an error using self.fail when '__class__' is assigned, matching the style used in other TypeChecker checks.

This change addresses issue #7724.

Author: lsrafael13

mypy/checker.py

@@ -8882,11 +8882,7 @@ def visit_assignment_stmt(self, s: AssignmentStmt) -> None:
             lv.accept(self)
             if isinstance(lv, MemberExpr):
                 if lv.name == '__class__':
-                    self.errors.report(
-                        lv.line,
-                        lv.column,
-                        "Assignment to '__class__' is unsafe and not allowed"
-                        )
+                    self.fail("Assignment to '__class__' is unsafe and not allowed", lv)
         self.lvalue = False
 
     def visit_name_expr(self, e: NameExpr) -> None: