title: Automatically create requirements.txt
body: <p>Sometimes I download the python source code from <code>github</code> and don't know how to install all the dependencies. If there is no <code>requirements.txt</code> file I have to create it by hands.<br />
  The question is:<br />
  Given the python source code directory is it possible to create <code>requirements.txt</code> automatically from the import section?</p>