* bpo-18748: Fix _pyio.IOBase destructor (closed case) (GH-13952)

_pyio.IOBase destructor now does nothing if getting the closed
attribute fails to better mimick _io.IOBase finalizer.

(cherry picked from commit 4f6f7c5)

* bpo-37223: test_io: silence destructor errors (GH-13954)

Implement also MockNonBlockWriterIO.seek() method.

(cherry picked from commit b589cef)

* bpo-37223, test_io: silence last 'Exception ignored in:' (GH-14029)

Use catch_unraisable_exception() to ignore 'Exception ignored in:'
error when the internal BufferedWriter of the BufferedRWPair is
destroyed. The C implementation doesn't give access to the
internal BufferedWriter, so just ignore the warning instead.

(cherry picked from commit 913fa1c)