I see the following kind of exceptions when running test_memoryio:

Exception ignored in: <function IOBase.__del__ at 0x7f7cd44f49b0>
Traceback (most recent call last):
  File "/home/antoine/cpython/default/Lib/_pyio.py", line 409, in __del__
    self.close()
  File "/home/antoine/cpython/default/Lib/_pyio.py", line 2152, in close
    if self.buffer is not None and not self.closed:
  File "/home/antoine/cpython/default/Lib/_pyio.py", line 2093, in buffer
    return self._buffer
AttributeError: 'StringIO' object has no attribute '_buffer'
Exception ignored in: <function IOBase.__del__ at 0x7f7cd44f49b0>
Traceback (most recent call last):
  File "/home/antoine/cpython/default/Lib/_pyio.py", line 409, in __del__
    self.close()
  File "/home/antoine/cpython/default/Lib/_pyio.py", line 2152, in close
    if self.buffer is not None and not self.closed:
  File "/home/antoine/cpython/default/Lib/_pyio.py", line 2093, in buffer
    return self._buffer
AttributeError: 'StringIO' object has no attribute '_buffer'
ok

It seems this could be related to issue18748.