tox: Set QT_QPA_PLATFORM_PLUGIN_PATH for smoke.

This fixes smoke tests on Windows (I think).

tox.ini

@@ -113,6 +113,8 @@ commands =
     {envpython} scripts/asciidoc2html.py {posargs}
 
 [testenv:smoke]
+# https://bitbucket.org/hpk42/tox/issue/246/ - only needed for Windows though
+setenv = QT_QPA_PLATFORM_PLUGIN_PATH={envdir}/Lib/site-packages/PyQt5/plugins/platforms
 passenv = DISPLAY XAUTHORITY HOME
 deps =
     -r{toxinidir}/requirements.txt