title: Error installing ipdb for Python 2.7 using virtualenv and pip
body: <p>When I tried to install <code>ipdb</code>, I had the following problem:</p>
  
  <pre><code>$ pip install ipdb
  Collecting ipdb
    Using cached ipdb-0.10.3.tar.gz
      Complete output from command python setup.py egg_info:
      error in ipdb setup command: Invalid environment marker: python_version &gt;= "3.3"
  </code></pre>
  
  <p>How can I install <code>ipdb</code> in the simplest way?</p>
  
  <p>(I use macOS Sierra 10.12.4, virtualenv 1.11.6, python 2.7.10, pip 9.0.1)</p>