Set PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION for pure python comformance test

python/tox.ini

diff --git a/python/tox.ini b/python/tox.ini
index 1600db2..baa96db 100644
--- a/python/tox.ini
+++ b/python/tox.ini
@@ -9,6 +9,7 @@
     cpp: LD_LIBRARY_PATH={toxinidir}/../src/.libs
     cpp: DYLD_LIBRARY_PATH={toxinidir}/../src/.libs
     cpp: PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=cpp
+    python: PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=python
 commands =
     python setup.py -q build_py
     python: python setup.py -q build