Yubikey Nano 4 not returned by u2f.list_devices()

[dahu33]

The following exception (which is silently ignored) prevent the Yubikey Nano 4 to be listed by u2f.list_devices() on MacOSX (not sure about the other OS):

Python 2.7:

Python 2.7.10 (default, Feb  7 2017, 00:08:15) 
[GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.34)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from u2flib_host import hid_transport
>>> hid_transport.list_devices()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/prousset/Library/Python/2.7/lib/python/site-packages/u2flib_host/hid_transport.py", line 81, in list_devices
    device.open()
  File "/Users/prousset/Library/Python/2.7/lib/python/site-packages/u2flib_host/hid_transport.py", line 116, in open
    self.handle.open_path(self.path)
  File "hid.pyx", line 72, in hid.device.open_path (hid.c:1909)
IOError: open failed

Python 3:

Python 3.6.1 (default, Apr  4 2017, 09:40:21) 
[GCC 4.2.1 Compatible Apple LLVM 8.1.0 (clang-802.0.38)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from u2flib_host import hid_transport
>>> hid_transport.list_devices()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.6/site-packages/u2flib_host/hid_transport.py", line 81, in list_devices
    device.open()
  File "/usr/local/lib/python3.6/site-packages/u2flib_host/hid_transport.py", line 116, in open
    self.handle.open_path(self.path)
  File "hid.pyx", line 72, in hid.device.open_path (hid.c:1909)
OSError: open failed