title: Print only the message on warnings
body: <p>I'm issuing lots of warnings in a validator, and I'd like to suppress everything in stdout except the message that is supplied to <code>warnings.warn()</code>.</p>
  <p>I.e., now I see this:</p>
  <pre class="lang-none prettyprint-override"><code>./file.py:123: UserWarning: My looong warning message
  some Python code
  </code></pre>
  <p>I'd like to see this:</p>
  <pre class="lang-none prettyprint-override"><code>My looong warning message
  </code></pre>
  <p><strong>Edit 2:</strong> Overriding <code>warnings.showwarning()</code> turned out to work:</p>
  <pre><code>def _warning(
      message,
      category = UserWarning,
      filename = '',
      lineno = -1):
      print(message)
  ...
  warnings.showwarning = _warning
  warnings.warn('foo')
  </code></pre>