title: Specifying optional dependencies in pypi python setup.py
body: <p>How do I specify optional dependencies in python's <code>setup.py</code> ?</p>
  
  <p>Here's my stab at specifying an optional dependency for an open source library of mine but it doesn't seem to do much.</p>
  
  <p><a href="https://github.com/od-eon/django-cherrypy/blob/master/setup.py" rel="noreferrer">https://github.com/od-eon/django-cherrypy/blob/master/setup.py</a></p>
  
  <p>Specifically <code>extra_requires</code> in this snippet:</p>
  
  <pre><code>setup(
      name='django-cherrypy',
      version='0.1',
      packages=packages,
      license='LICENSE',
      description='cherrypy, running under django',
      long_description=open('README.md').read(),
      author='Calvin Cheng',
      author_email='calvin@calvinx.com',
      install_requires=['cherrypy-wsgiserver'],
      extra_requires=['newrelic'],
      url='https://github.com/od-eon/django-cherrypy',
  )
  </code></pre>
  
  <p>Suggestions?</p>