Fix assignment to __class__ check to use errors.report() in VarAssignVisitor (Fixes #7724)

Replaces previous incorrect call to self.chk.fail() with self.errors.report(), resolving AttributeError.
Ensures assignment to __class__ is properly reported as an error during instance attribute assignments.

Author: lsrafael13

mypy/checker.py

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