Travis CI: Add Python 3.7 to the testing
Travis CI: Add Python 3.7 to the testing

Python 3.7 added in alignment with travis-ci/travis-ci#9069
mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext,mwouts/jupytext