bpo-18748: _pyio.IOBase emits unraisable exception

[vstinner]

In development (-X dev) mode and in a debug build, IOBase finalizer
of the _pyio module now logs the exception if the close() method
fails. The exception is ignored silently by default in release build.

test_io: test_error_through_destructor() now uses
support.catch_unraisable_exception() rather than capturing stderr.

https://bugs.python.org/issue18748

[vstinner]

Ah, test_io failed on Travis CI. Example:

FAIL: test_error_through_destructor (test.test_io.CBufferedReaderTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/python/cpython/Lib/test/test_io.py", line 1107, in test_error_through_destructor
    self.assertIsNotNone(cm.unraisable)
AssertionError: unexpectedly None