Work around tox bug.

python/tox.ini

@@ -6,6 +6,10 @@ envlist =
 usedevelop=true
 passenv = CC
 setenv =
+    # Dummy entry works around bug where tox fails for empty "setenv" section
+    # (since cpp lines aren't used for py builds).
+    # https://bitbucket.org/hpk42/tox/issues/190/generative-setenv-fails-if-there-s-only
+    DUMMY=dummy
     cpp: LD_LIBRARY_PATH={toxinidir}/../src/.libs
     cpp: DYLD_LIBRARY_PATH={toxinidir}/../src/.libs
     cpp: PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=cpp