tox: tox.ini fixes for Ubuntu Trusty/tox 1.6

- Don't use old PYTHONPATH in tox.ini.
  PYTHONPATH should be unset anyways, so it's okay to ignore the old value.

- Don't use config interpolation for unittests.
  This shows a "command not found" error for some reason.

tox.ini

@@ -16,8 +16,10 @@ usedevelop = true
 
 [testenv:unittests]
 setenv = QT_QPA_PLATFORM_PLUGIN_PATH={envsitepackagesdir}/PyQt5/plugins/platforms
+# We don't use {[testenv:mkvenv]commands} here because that seems to be broken
+# on Ubuntu Trusty.
 commands =
-    {[testenv:mkvenv]commands}
+    {envpython} scripts/link_pyqt.py --tox {envdir}
     {envpython} -m unittest
 
 [testenv:coverage]
@@ -37,7 +39,7 @@ commands =
 
 [testenv:pylint]
 skip_install = true
-setenv = PYTHONPATH={toxinidir}/scripts:{env:PYTHONPATH:}
+setenv = PYTHONPATH={toxinidir}/scripts
 deps =
     -rrequirements.txt
     astroid==1.3.6