Add Python 3 compatibility.

ci/before_install_nonjython.sh

@@ -1,5 +1,8 @@
 #!/bin/bash
 set -e
 
-cat requirements-python.txt >> requirements.txt
+if [ -f requirements-python-${TRAVIS_PYTHON_VERSION}.txt ]
+then
+    cat requirements-python-${TRAVIS_PYTHON_VERSION}.txt >> requirements.txt
+fi
 ln -s $VIRTUAL_ENV $HOME/myvirtualenv