title: tox: how to specify python micro version
body: <p>In the example below, the python version is 2.7.6.</p>
  
  <ul>
  <li>Why is it 2.7.6, and not the latest 2.7.15?</li>
  <li>How do I specify 2.7.15?</li>
  </ul>
  
  <p>Set up</p>
  
  <pre><code>$ tox --version
  2.7.0 imported from /home/obk/anaconda3/lib/python3.6/site-packages/tox/__init__.py
  
  $ ls
  tox.ini
  
  $ cat tox.ini
  [tox]
  envlist = py27
  skipsdist = True
  
  [testenv]
  commands = python -V
  </code></pre>
  
  <p>Running it:</p>
  
  <pre><code>$ tox -r
  py27 recreate: /home/obk/repos/test/.tox/py27
  py27 installed:
  py27 runtests: PYTHONHASHSEED='4191112007'
  py27 runtests: commands[0] | python -V
  Python 2.7.6
  ___________________________________ summary ___________________________________________
    py27: commands succeeded
    congratulations :)
  </code></pre>
  
  <p>The reason why I want the latest 2.7.15 is because my project has a dependency (<code>my_foo</code> -> <code>nbconvert</code> -> <code>tornado</code>) that is now using <code>ssl.create_default_context</code> which was <a href="https://docs.python.org/2/library/ssl.html#ssl.create_default_context" rel="nofollow noreferrer">added in 2.7.9</a>.</p>