Set PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION for pure python comformance test

python/tox.ini

@@ -9,6 +9,7 @@ setenv =
     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