ci(Travis): Fix dist for Python >= 3.7
ci(Travis): Fix dist for Python >= 3.7

See https://docs.travis-ci.com/user/languages/python/#python-37-and-higher

stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila