BLD: Fix Travis CI Python 3.7 on macOS, build on Python 3.8 on macOS and Linux (#252)
BLD: Fix Travis CI Python 3.7 on macOS, build on Python 3.8 on macOS and Linux (#252)

* BLD: Update Travis CI to force Python 3.7 on macOS

* Pin osx_image to lower xcode version

* Support Python 3.8 in Travis-CI

tkphd/pycalphad,tkphd/pycalphad,tkphd/pycalphad