It looks like that no Python version is available for macOS:
3.4 is not installed; attempting download
Downloading archive: https://storage.googleapis.com/travis-ci-language-archives/python/binaries/osx/10.13/x86_64/python-3.4.tar.bz2
0.17s$ curl -sSf -o python-3.4.tar.bz2 ${archive_url}
curl: (22) The requested URL returned error: 404
Unable to download 3.4 archive. The archive may not exist. Please consider a different version.
This is for Python 2.7, 3.4 through 37, PyPy and PyPy3.
Is this a known limitation? How can I make this work? When will Python be supported on Mac builds?
Python is one of those languages that are both very popular and run across all popular platforms. It is important to be able to run tests across platforms.