title: Tox and django_toolbar: ImportError
body: <p>When trying to use Tox to better streamline testing across multiple environments, I'm runing into the following error when testing for Python 3.4:</p>
  
  <p><code>ImportError: No module named 'debug_toolbar'</code></p>
  
  <p>However, <code>django-debug-toolbar==1.3.0</code> is listed in my <code>requirements.txt</code> file, and my <code>tox.ini</code> file looks as follows:</p>
  
  <pre><code>[tox]
  envlist = py27,py34
  skipsdist = True
  
  [testenv]
  deps = -r{toxinidir}/requirements.txt
  setenv =
      PYTHONPATH = {toxinidir}:{toxinidir}
  commands = python manage.py test
  </code></pre>
  
  <p>It seems as if it is not properly installing the requirements.. Oddly enough, the py27 environment does not throw this error, and is able to perform the tests just fine.</p>
  
  <p>What could be causing this?</p>
  
  <p>EDIT: for reference, here's my current setup. When I run tox the first time (i.e. without .tox), it works fine, but any times after that it fails. The <code>.tox</code> directory does seem to get build up correctly; all dependencies are installed in <code>.tox/py34/lib/python3.4/site-packages</code>. Compared to earlier, I now also uninstalled <code>django</code>, and indeed that is now the first dependency that fails.</p>
  
  <pre><code>(venv)joost@thorin:myproject/ (master✗) % rm -rf .tox
  (venv)joost@thorin:myproject/ (master✗) % cat tox.ini
  [tox]
  envlist = py27,py34
  skipsdist = True
  
  [testenv]
  deps = -r{toxinidir}/requirements.txt
  commands = python manage.py test
  
  
  (venv)joost@thorin:myproject/ (master✗) % tox
  py27 create: /Users/Joost/myproject/.tox/py27
  py27 installdeps: -r/Users/Joost/myproject/requirements.txt
  py27 runtests: PYTHONHASHSEED='4248725049'
  py27 runtests: commands[0] | python manage.py test
  Creating test database for alias 'default'...
  ................
  ----------------------------------------------------------------------
  Ran 16 tests in 0.093s
  
  OK
  Destroying test database for alias 'default'...
  py34 create: /Users/Joost/myproject/.tox/py34
  py34 installdeps: -r/Users/Joost/myproject/requirements.txt
  py34 runtests: PYTHONHASHSEED='4248725049'
  py34 runtests: commands[0] | python manage.py test
  Creating test database for alias 'default'...
  ................
  ----------------------------------------------------------------------
  Ran 16 tests in 0.093s
  
  OK
  Destroying test database for alias 'default'...
  ____________________________________________________________________ summary _____________________________________________________________________
    py27: commands succeeded
    py34: commands succeeded
    congratulations :)
  
  
  (venv)joost@thorin:myproject/ (master✗) % tox
  py27 runtests: PYTHONHASHSEED='3259360769'
  py27 runtests: commands[0] | python manage.py test
  Creating test database for alias 'default'...
  ................
  ----------------------------------------------------------------------
  Ran 16 tests in 0.088s
  
  OK
  Destroying test database for alias 'default'...
  py34 runtests: PYTHONHASHSEED='3259360769'
  py34 runtests: commands[0] | python manage.py test
  Traceback (most recent call last):
    File "manage.py", line 8, in &lt;module&gt;
      from django.core.management import execute_from_command_line
  ImportError: No module named 'django'
  ERROR: InvocationError: '/Users/Joost/myproject/.tox/py34/bin/python manage.py test'
  ____________________________________________________________________ summary _____________________________________________________________________
    py27: commands succeeded
  ERROR:   py34: commands failed
  (venv)joost@thorin:myproject/ (master✗) % 
  </code></pre>