I ran the test with the -v option flag. The malloc error don't happen at the same place the hang up happens. The first one happens at:

test_readline (test.test_io.PyIOTest) ... ok
test_unbounded_file (test.test_io.PyIOTest) ... skipped 'test can only run in a 32-bit address space'
test_with_open (test.test_io.PyIOTest) ... ok
test_buffering (test.test_io.CBufferedReaderTest) ... ok
test_constructor (test.test_io.CBufferedReaderTest) ... python.exe(24083) malloc: *** mmap(size=9223372036854775808) failed (error code=12)
*** error: can't allocate region
*** set a breakpoint in malloc_error_break to debug
ok
test_context_manager (test.test_io.CBufferedReaderTest) ... ok
test_detach (test.test_io.CBufferedReaderTest) ... ok

And the second one happens at:

test_readlines (test.test_io.PyBufferedReaderTest) ... ok
test_repr (test.test_io.PyBufferedReaderTest) ... ok
test_threads (test.test_io.PyBufferedReaderTest) ... skipped "resource 'cpu' is not enabled"
test_constructor (test.test_io.CBufferedWriterTest) ... python.exe(24083) malloc: *** mmap(size=9223372036854775808) failed (error code=12)
*** error: can't allocate region
*** set a breakpoint in malloc_error_break to debug
ok
test_context_manager (test.test_io.CBufferedWriterTest) ... ok
test_destructor (test.test_io.CBufferedWriterTest) ... ok

And finally, the hang:

test_io_after_close (test.test_io.CMiscIOTest) ... ok
test___all__ (test.test_io.PyMiscIOTest) ... ok
test_abc_inheritance (test.test_io.PyMiscIOTest) ... ok
test_abc_inheritance_official (test.test_io.PyMiscIOTest) ... ok
test_abcs (test.test_io.PyMiscIOTest) ... ok
test_attributes (test.test_io.PyMiscIOTest) ... ok
test_blockingioerror (test.test_io.PyMiscIOTest) ... ok
test_io_after_close (test.test_io.PyMiscIOTest) ... ok
test_interrupted_write_buffered (test.test_io.CSignalsTest) ... <hang>