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The motivation came from reading Brett Cannon's article : https://snarky.ca/how-to-use-your-project-travis-to-help-test-python-itself/ .
Trying to activate the newest Python versions on CI jobs is in most cases a win-win situation:
 - if everything works fine, there is nothing to worry about.
 - if an issue is spotted, it is good to know about it to fix it on your side or to open a bug on CPython ( https://bugs.python.org/ ).

Also, if a failure is spotted, you can use the allow_failures option in your matrix build (more information about this in the link above).

More information about how this PR happened : https://github.com/SylvainDe/CIthon .

More information about the various Python versions available on Travis:
 - Official documentation:
    https://docs.travis-ci.com/user/languages/python/ .
 - Tiny projects to see which Python versions are actually working on Travis:
    https://travis-ci.org/SylvainDe/ContinuousIntegrationPythonVersions .

More information about the Python release schedule:
 - Python 3.9: https://www.python.org/dev/peps/pep-0596/ .
 - Python 3.8: https://www.python.org/dev/peps/pep-0569/ .
 - Python 3.7: https://www.python.org/dev/peps/pep-0537/ .
 - Python 3.6: https://www.python.org/dev/peps/pep-0494/ .

Additional words from important Python devs about the importance of testing the 3.8 version:
 - Guido van Rossum:
    https://twitter.com/gvanrossum/status/1167659428142825474
 - Raymond Hettinger:
    https://twitter.com/raymondh/status/1167459315508113408
    https://twitter.com/raymondh/status/1153713205245595648
 - Łukasz Langa:
    https://twitter.com/llanga/status/1167567333059104768
    https://twitter.com/llanga/status/1167370321902673920
 - Paul Ganssle:
    https://twitter.com/pganssle/status/1123019315035496453

Additional Travis tips:
 - to collect more data on the versions being used, you can use:
```
before_script:
- python -VV
- python -c "import sys; print(sys._git)" || true
```
 - to get your code run regularly, you can use Cron Jobs:
    https://docs.travis-ci.com/user/cron-jobs/