title: run python script with subprocess fails on ImportError
body: <p>My files' tree is something like:</p>
  <pre><code>package1\
      __init__.py
      parent_script.py
  package2\
      __init__.py
      child_script.py
      module.py
  </code></pre>
  <p>I'm trying to execute <code>child_script.py</code> as a subprocess from <code>parent_script.py</code>.</p>
  <p><code>child_script.py</code> uses import from <code>package2.module.py</code>:</p>
  <pre><code>ROOT_DIR = os.path.abspath(os.path.join(os.path.dirname(__file__), '..'))
  sys.path.append(ROOT_DIR)
  from package2.module import func1, func2
  </code></pre>
  <p>I'm getting <code>ImportError: No module named..</code> when running <code>child_script.py</code> as a subprocess, <strong>but it runs well independently.</strong></p>
  <p>Where am I wrong?</p>