[3.11] pythongh-87846: test_io: Ignore OpenWrapper in test___all__

The deprecated `OpenWrapper` is added to the module on demand
in `__getattr__`, so it might or might not show up in `dir(io)`
depending on whether e.g. `test_openwrapper` was run.

Add in to `not_exported` so that check__all__ ignores it
when it exists.

This deterministically fails on some refleaks buildbots (but
not when the test is re-run, so it's only marked as a warning).
Locally, it can be reproduced by running `test_openwrapper` and
`test__all__`, twice:

    ./python -m test test_io test_io -m '*test_[o_][p_][ea][nl][wl]*' -v

Author: encukou

Lib/test/test_io.py

@@ -4504,6 +4504,7 @@ class CMiscIOTest(MiscIOTest):
     io = io
     name_of_module = "io", "_io"
     extra_exported = "BlockingIOError",
+    not_exported = "OpenWrapper"  # deprecated, added on demand
 
     def test_readinto_buffer_overflow(self):
         # Issue #18025
@@ -4570,7 +4571,10 @@ class PyMiscIOTest(MiscIOTest):
     io = pyio
     name_of_module = "_pyio", "io"
     extra_exported = "BlockingIOError", "open_code",
-    not_exported = "valid_seek_flags",
+    not_exported = (
+        "valid_seek_flags",
+        "OpenWrapper",   # deprecated, added on demand
+    )
 
 
 @unittest.skipIf(os.name == 'nt', 'POSIX signals required for this test.')