pythongh-138360: Fix test_free_threading for IO objects (python#138359)

Fix typo in test_io.py

Author: uhx

Lib/test/test_free_threading/test_io.py

@@ -41,7 +41,7 @@ def writelines(barrier, b, *ignore):
         def truncate(barrier, b, *ignore):
             barrier.wait()
             try: b.truncate(0)
-            except: BufferError  # ignore exported buffer
+            except BufferError: pass  # ignore exported buffer
 
         def read(barrier, b, *ignore):
             barrier.wait()