title: python3 tkinter ubuntu trusty does not work under virtual environment
body: <p>I installed python3-tk on an ubuntu trusty docker container.(apt-get install python3-tk)</p>
  
  <pre><code>&gt; python3
  Python 3.4.3 (default, Oct 14 2015, 20:28:29) 
  [GCC 4.8.4] on linux
  Type "help", "copyright", "credits" or "license" for more information.
  &gt;&gt;&gt; import tkinter
  &gt;&gt;&gt; import tkinter as tk
  &gt;&gt;&gt; 
  </code></pre>
  
  <p>So far so good. However, for different reasons, I need to run a loaded python3 virtualenv in same container.
  When I activate the virtual environment:</p>
  
  <pre><code>(env_py34)root@8a7953c24d4f:/home# python
  Python 3.4.3 (default, Oct 14 2015, 20:28:29) 
  [GCC 4.8.4] on linux
  Type "help", "copyright", "credits" or "license" for more information.
  &gt;&gt;&gt; import tkinter as tk
  Traceback (most recent call last):
    File "/usr/lib/python3.4/tkinter/__init__.py", line 39, in &lt;module&gt;
      import _tkinter
  ImportError: No module named '_tkinter'
  
  During handling of the above exception, another exception occurred:
  
  Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
    File "/usr/lib/python3.4/tkinter/__init__.py", line 41, in &lt;module&gt;
      raise ImportError(str(msg) + ', please install the python3-tk package')
  ImportError: No module named '_tkinter', please install the python3-tk package
  &gt;&gt;&gt; 
  </code></pre>
  
  <p>Versions of python are identical.
  How come the virtual environment did not inherit python3-tk from the python3 installation? How can I install python3-tk inside the virtual environment?</p>