Create io.BytesIO() in destructor of test_io.py

WrapperTest.test_create_at_shutdown_with_encoding fails with something
like:

```
Exception ignored in: <function C.__del__ at 0x101895ae0>
Traceback (most recent call last):
  File "x.py", line 10, in __del__
  File "/Users/sgross/Projects/nogil/Lib/_pyio.py", line 2030, in __init__
  File "/Users/sgross/Projects/nogil/Lib/_pyio.py", line 1021, in seekable
ValueError: I/O operation on closed file.
```

The problem is that the GC calls the finalizer on `self.buf` before the
`__del__` on `class C`. I don't think Python guarantees any ordering on
destructors.

Lib/test/test_io.py

@@ -3542,10 +3542,8 @@ def _check_create_at_shutdown(self, **kwargs):
             codecs.lookup('utf-8')
 
             class C:
-                def __init__(self):
-                    self.buf = io.BytesIO()
                 def __del__(self):
-                    io.TextIOWrapper(self.buf, **{kwargs})
+                    io.TextIOWrapper(io.BytesIO(), **{kwargs})
                     print("ok")
             c = C()
             """.format(iomod=iomod, kwargs=kwargs)