The error went away when I commented out following line.

Lib/unittest/case.py(133)
        self.exception = exc_value.with_traceback(None)

I found this by brute force.... I noticed that
test_tar_pipe_open_read_error_v2.py starts to fail from r75241.
It stores exc_value into _AssertRaisesContext#exception.

And when the exception was re-raised like this, (_Stream#_read)

            try:
                buf = self.cmp.decompress(buf)
            except IOError:
                raise ReadError("invalid compressed data")

IOError will be set to ReadError's *__cause__* attribute.
ReadError's traceback is cleared in unittest, but IOError's
traceback is not. It contains reference to frame which contains
_AssertRaisesContext object. And ReadError will be set to
_AssertRaisesContext's *exception* attribute, cyclic reference
is made. It cannot be freed without GC.

# gc.get_referents() was really helpful.

I'll attach the test script "test_assert_raises.py" to reproduce
the error. The result should be like this.

E:\python-dev>py3k test_assert_raises.py
--------------------------
[('foo 2',), None, RuntimeError('foo 1',)]
RuntimeError('foo 1',)
[('foo 1',), <traceback object at 0x00BD2738>]
<traceback object at 0x00BD2738>
--------------------------
.
----------------------------------------------------------------------
Ran 1 test in 0.090s

OK
===========================
gc: collectable <cell 00BD0B28>
gc: collectable <tuple 00BD0B60>
gc: collectable <function 00BCFE78>
gc: collectable <tuple 00BD0AF0>
gc: collectable <function 00BCFED8>
gc: collectable <tuple 00BD0BD0>
gc: collectable <function 00BCFF38>
gc: collectable <tuple 00BD0C08>
gc: collectable <function 00BCFF98>
gc: collectable <tuple 00BD0C40>
gc: collectable <function 00BD4038>
gc: collectable <tuple 00BD0C78>
gc: collectable <function 00BD4098>
gc: collectable <tuple 00BD0CB0>
gc: collectable <function 00BD40F8>
gc: collectable <function 00BD4158>
gc: collectable <type 00C07550>
gc: collectable <dict 00B1FAB8>
gc: collectable <getset_descriptor 00BD2678>
gc: collectable <getset_descriptor 00BD26B8>
gc: collectable <tuple 00AC53F8>
gc: collectable <frame 00C08858>
gc: collectable <method 00AC43F8>
gc: collectable <frame 00C09628>
gc: collectable <tuple 00BD0038>
gc: collectable <frame 00C097D0>
gc: collectable <frame 00C09978>
gc: collectable <frame 00C0C1B8>
gc: collectable <frame 00C0CEF0>
gc: collectable <tuple 00BD0E38>
gc: collectable <frame 00C0D098>
gc: collectable <method 00AC43B8>
gc: collectable <frame 00C0D810>
gc: collectable <_AssertRaisesContext 00BD0F18>
gc: collectable <frame 00C0DF18>
gc: collectable <tuple 00BD0EA8>
gc: collectable <RuntimeError 00B9FC50>
gc: collectable <traceback 00BD2738>
gc: collectable <tuple 00BD0F50>
gc: collectable <ValueError 00B9FC98>
gc: collectable <dict 00B22818>
gc: collectable <method 00AC4338>
gc: collectable <tuple 00BD0B98>
gc: collectable <tuple_iterator 00BD0E70>
gc: collectable <map 00BD0EE0>
gc: collectable <list 00B21938>
===========================
gc: 46 uncollectable objects at shutdown:
[45415 refs]