title: RHEL7 import tkinter failed inside virtualenv
body: <p>On Redhat 7 and python3.6</p>
  
  <p>I can import tkinter without any error:</p>
  
  <pre><code>Python 3.6.5 (default, Jul 25 2018, 21:22:33)
  [GCC 4.8.5 20150623 (Red Hat 4.8.5-28)] on linux
  Type "help", "copyright", "credits" or "license" for more information.
  &gt;&gt;&gt; import tkinter
  &gt;&gt;&gt;
  </code></pre>
  
  <p>Within a fresh virtualenv</p>
  
  <pre><code>Python 3.6.4 (default, Jul 20 2018, 12:22:32)
  [GCC 4.8.5 20150623 (Red Hat 4.8.5-28)] on linux
  Type "help", "copyright", "credits" or "license" for more information.
  &gt;&gt;&gt; import tkinter
  Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
    File "/ocr1/CRNN_Tensorflow/tkinter/__init__.py", line 36, in &lt;module&gt;
      import _tkinter # If this fails your Python may not be configured for Tk
  ModuleNotFoundError: No module named '_tkinter'
  &gt;&gt;&gt;
  </code></pre>
  
  <p>P.S. It's an entire new virtualenv, created by <code>virtualenv &lt;name_of_env&gt;</code> command.</p>