title: Proper Python packaging with external package dependencies
body: <p>Let's suppose I have a project that depends on packages that should only be installed via the systems package manager (e.g. bindings to C libraries), so that <code>install_requires</code> is not working.</p>
  
  <p>Currently, I try to import said packages in <code>setup.py</code> and call <code>sys.exit()</code> with a positive exit code if the import fails. Is this good practice or are there better ways to accomplish this?</p>