tl; dr Travis CI issues are now resolved thanks to Ernest!
During the last 3 months, the Travis CI job failed randomly to report the build status to GitHub pull requests: https://github.com/python/core-workflow/issues/371
I discovered that travis-ci.org uses the legacy GitHub API, whereas there is a new travis-ci.com website which uses the new GitHub Action API. The migration started in May 2018, is still in the beta phase, and must be done manually:
Two weeks ago, Ernest W. Durbin III migrated the GitHub "python" organization from travis-ci.org to travis-ci.com, and also migrated the cpython project to travis-ci.com (the migration requires to migrate each project individually).
Since the migration, I didn't notice the "Travis CI doesn't report the build status" issue anymore on new pull requests. Great!