title: Check if a python function is builtin
body: <p>How can I check whether a python function is a built-in function?</p>
  
  <p>I want something like this:</p>
  
  <pre><code>def abc(x):
    return x
  
  is_builtin(abc)
  </code></pre>
  
  <p>should return False</p>
  
  <pre><code>import os
  is_builtin(os.path.join)
  </code></pre>
  
  <p>should return True</p>
  
  <pre><code>from os import *
  is_builtin(path.join)
  </code></pre>
  
  <p>should return True</p>
  
  <p>Any help would be appreciated</p>