gh-133581: Fix refleak in t-string AST unparsing (#133724)

Author: JelleZijlstra

Python/ast_unparse.c

@@ -749,6 +749,7 @@ append_templatestr(PyUnicodeWriter *writer, expr_ty e)
             goto error;
         }
     }
+    _PyArena_Free(arena);
 
     return 0;