Keep the existing PYTHONPATH in tox

Without this the tests could not be run on systems that already use
PYTHONPATH (like NixOS).

Author: ktosiek

generate_configurations.py

@@ -49,7 +49,7 @@ def is_pypy(self):
     deps =
     %(deps)s
     setenv =
-         PYTHONPATH = {toxinidir}
+         PYTHONPATH = {toxinidir}:{env:PYTHONPATH:}
          UID = %(uid)s
     """)