title: pylint complains on py.test: "Module 'pytest' has no 'raises' member"
body: <p>With the following code:</p>
  
  <pre><code>import pytest
  def test_a():
      with pytest.raises(Exception):
          1/0
  </code></pre>
  
  <p>If I run pylint on it, it will make a complain that "raises" is not a member of module pytest:</p>
  
  <pre><code>E:  3,9:test_a: Module 'pytest' has no 'raises' member
  </code></pre>
  
  <p>Which is obviously not true. Any idea why pylint is making such a mistake? Is this a known bug?</p>
  
  <p>py.test version:</p>
  
  <pre><code>&gt; py.test --version
  This is py.test version 2.2.3, imported from C:\Python27\lib\site-packages\pytest.pyc
  </code></pre>
  
  <p>PyLint version:</p>
  
  <pre><code>&gt; pylint --version
  No config file found, using default configuration
  pylint 0.25.1,
  astng 0.23.1, common 0.57.1
  Python 2.7.2 (default, Jun 24 2011, 12:22:14) [MSC v.1500 64 bit (AMD64)]
  </code></pre>