gh-127190: Fix local_setattro() error handling (#127366)

Don't make the assumption that the 'name' argument is a string. Use
repr() to format the 'name' argument instead.

Lib/test/test_threading_local.py

@@ -208,6 +208,21 @@ class BaseLocalTest:
 
         _testcapi.join_temporary_c_thread()
 
+    @support.cpython_only
+    def test_error(self):
+        class Loop(self._local):
+            attr = 1
+
+        # Trick the "if name == '__dict__':" test of __setattr__()
+        # to always be true
+        class NameCompareTrue:
+            def __eq__(self, other):
+                return True
+
+        loop = Loop()
+        with self.assertRaisesRegex(AttributeError, 'Loop.*read-only'):
+            loop.__setattr__(NameCompareTrue(), 2)
+
 
 class ThreadLocalTest(unittest.TestCase, BaseLocalTest):
     _local = _thread._local