title: nxt-python error: usb.core.NoBackendError
body: <p>I have Python 2 and Python 3 installed on my system.</p>
  
  <p>I run the following script:</p>
  
  <pre><code>#!/usr/bin/env python
  
  import nxt.locator
  from nxt.motor import *
  
  def spin_around(b):
      m_left = Motor(b, PORT_B)
      m_left.turn(100, 360)
      m_right = Motor(b, PORT_C)
      m_right.turn(-100, 360)
  
  b = nxt.locator.find_one_brick()
  spin_around(b)
  </code></pre>
  
  <p>with the command: <code>python spin.py</code> which uses Python 2, as expected.</p>
  
  <p>However, when  I run this command I get the following error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "spin.py", line 12, in &lt;module&gt;
      b = nxt.locator.find_one_brick()
    File "/Library/Python/2.7/site-packages/nxt/locator.py", line 112, in find_one_brick
      for s in find_bricks(host, name, silent, method):
    File "/Library/Python/2.7/site-packages/nxt/locator.py", line 43, in find_bricks
      for s in socks:
    File "/Library/Python/2.7/site-packages/nxt/usbsock.py", line 84, in find_bricks
      for bus in usb.busses():
    File "/Library/Python/2.7/site-packages/usb/legacy.py", line 353, in busses
      sorted(core.find(find_all=True), key=lambda d: d.bus),
    File "/Library/Python/2.7/site-packages/usb/core.py", line 1263, in find
      raise NoBackendError('No backend available')
  usb.core.NoBackendError: No backend available
  </code></pre>
  
  <p>When I run this script my NXT is plugged into my computer by USB and my NXT is on. Additionally, I have installed pyUSB (correctly, I think). </p>
  
  <p>Why could this error be occurring?</p>