Issue 31976: Segfault when closing BufferedWriter from a different thread

To reproduce:
```
import threading
import io
import time
import _pyio

class MyFileIO(io.FileIO):
    def flush(self):
        # Simulate a slow flush. Slow disk, etc.
        time.sleep(0.25)
        super().flush()

raw = MyFileIO('test.dat', 'wb')
#fp = _pyio.BufferedWriter(raw)
fp = io.BufferedWriter(raw)
t1 = threading.Thread(target=fp.close)
t1.start()
time.sleep(0.1) # Ensure t1 is sleeping in fp.close()/raw.flush()
fp.write(b'test')
t1.join()
```

_pyio.BufferedWriter ignores the error completely rather than throwing a ValueError("write to closed file").

New changeset 9703f092abc0259926d88c7855afeae4a78afc7d by Antoine Pitrou (benfogle) in branch 'master':
bpo-31976: Fix race condition when flushing a file is slow. (#4331)
https://github.com/python/cpython/commit/9703f092abc0259926d88c7855afeae4a78afc7d

Thank you for posting this Benjamin.  As I said on the PR, I don't think I want to backport this to 3.6, as it is always delicate to reason about subclassing and threads.