https://docs.travis-ci.com/user/languages/python/#specifying-python-versions has a (supposedly) comprensive list of valid values for the
python: field for normal Python.
There’s, however, no such list in https://docs.travis-ci.com/user/languages/python/#pypy-support : it only mentions
pypy3.5. Distro-specific articles don’t have any comprehensive lists, either.
Experiments show that there are no validation checks in the build logic, it just blindly tries to fetch a tarball with the corresponding name from https://s3.amazonaws.com/travis-python-archives/binaries/ubuntu/16.04/x86_64/ . Since that site doesn’t provide directory listings, there’s no way to get a list of available files – hence valid values for