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>>> io.UnsupportedOperation.__new__(io.UnsupportedOperation)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: ValueError.__new__(UnsupportedOperation) is not safe, use OSError.__new__()
Looking at the mro of io.UnsupportedOperation
(io.UnsupportedOperation, ValueError, OSError, Exception, BaseException, object'>)
Shows that ValueError.__new__ is getting called, but will not construct 
an instance of a subtype of OSError
Switching the order of ValueError and OSError fixes this error (patch attached), but doesn't fix the underlying problem.
Why doesn't io.UnsupportedOperation() fail, when
UnsupportedOperation.__new__ does?
io.UnsupportedOperation() actually calls OSError.__new__ 
via the internal tp_new slot.
In other words UnsupportedOperation->tp_new != UnsupportedOperation.__new__
This should not happen, it looks like the logic in inherit_special()
in typeobject.c may be faulty.

>>> io.UnsupportedOperation.__new__(io.UnsupportedOperation)
behaves correctly now (rev d9c98730e2e8)
This bug was (I believe) caused somehow by an error in OSError_new() which did not initialize self->args if OSError_init() was not called.
Here is a crash which exploited the lack of initialization:
(Works on rev 2a142141e5fd)
>>> class C(ValueError, OSError):pass
... 
>>> c = OSError.__new__(C)
>>> str(c)
Segmentation fault (core dumped)
This has already been fixed, so I'm closing this issue.