bpo-37223: io.BufferedWrite.close() don't call flush() if already closed

[vstinner]

The close() method of io buffered files no longer call flush()
if the file is already closed.

https://bugs.python.org/issue37223

[vstinner]

This PR fix the following 2 errors when running test_io:

test_writer_close_error_on_close (test.test_io.CBufferedRWPairTest) ... Exception ignored in: <_io.BufferedWriter>
Traceback (most recent call last):
  File "/home/vstinner/prog/python/master/Lib/unittest/case.py", line 611, in _callTestMethod
    method()
ValueError: flush of closed file
ok

test_destructor (test.test_io.CBufferedRandomTest) ... Exception ignored in: <_io.BufferedRWPair object at 0x7fc9d1c230b0>
Traceback (most recent call last):
  File "/home/vstinner/prog/python/master/Lib/test/support/__init__.py", line 1627, in gc_collect
    gc.collect()
ValueError: flush of closed file
Exception ignored in: <_io.BufferedWriter>
Traceback (most recent call last):
  File "/home/vstinner/prog/python/master/Lib/test/support/__init__.py", line 1627, in gc_collect
    gc.collect()
ValueError: flush of closed file
ok

[vstinner]

I rebased my PR, added an unit test and I also fixed the pure Python implementation.

[vstinner]

This PR changes the behavior of the io module for a corner case. I merged PR #14028 instead to only fix an error when running test_io. IMHO the current behavior makes sense and ensures that text, buffered and fileio states are consistent, especially for the closed attribute. I close this PR.