Example of failure of a backport from 3.8 to 3.7, PR 17577:
https://github.com/python/cpython/pull/17577

"""
$ python --version

Python 3.6.9

$ pip --version

pip 19.3.1 from /home/travis/virtualenv/python3.6.9/lib/python3.6/site-packages/pip (python 3.6)
before_install.1

0.00s$ set -e

$ pyenv global 3.7.1

pyenv: version `3.7.1' not installed
"""

Travis CI logs:
https://travis-ci.org/python/cpython/jobs/624160244

Thread on python-dev:
https://mail.python.org/archives/list/[email protected]/thread/YCTLWAYIC44YTVGNN4EDLWKMER2LAPDA/

New changeset 94d2c8df1a7657015a2fcdb4c4d43392f91f8348 by Inada Naoki in branch 'master':
bpo-39035: travis: Don't use beta group (GH-17602)
https://github.com/python/cpython/commit/94d2c8df1a7657015a2fcdb4c4d43392f91f8348

New changeset 5c5d8f63d7d235e557ad20e7d722b22772681759 by Inada Naoki in branch '3.8':
bpo-39035: travis: Don't use beta group (GH-17603)
https://github.com/python/cpython/commit/5c5d8f63d7d235e557ad20e7d722b22772681759

New changeset be7489cb43e25b6d8bfa077589c18cc0a2367efd by Inada Naoki in branch '3.7':
bpo-39035: travis: Don't use beta group (GH-17604)
https://github.com/python/cpython/commit/be7489cb43e25b6d8bfa077589c18cc0a2367efd

New changeset de4481339dec395d70e350aa2e22d7990d2b3635 by Inada Naoki in branch '2.7':
bpo-39035: travis: Don't use beta group (GH-17605)
https://github.com/python/cpython/commit/de4481339dec395d70e350aa2e22d7990d2b3635

INADA-san: do you want to close this issue since you pushed changes, or do you prefer to wait to see if it's issue is fixed before closing it? (wait a few days and look at Travis CI jobs)

3.5 uses:

language: c
dist: trusty
sudo: false
group: beta

3.6 uses:

language: c
dist: trusty
sudo: false
group: beta

These branches still get pull requests for security fixes. Maybe it's worth it to also update these branches? Maybe even upgrade them to Ubuntu Xenial?

I already diagnosed this in https://mail.python.org/archives/list/[email protected]/message/3Z4FNPEFTFTYDX6RYOQ54UKOVLQAWUEU/:

* https://travis-ci.org/python/cpython/jobs/616384134 looks for `pythonX.Y` but doesn't account for the fact that it could be Pyenv's stub
* https://travis-ci.org/python/cpython/builds/616384157 is trying to use Python version(s) that are not guaranteed to be preinstalled in the VM for all `language:`s.

I could fix this but I need to know what the hard requirements are to pick an optimal configuration.

* Is the (preinstalled only for `language: c`) `clang 7.0.0` required? Would 3.4-4.0 (available from xenial distro) do?
* Is `xenial` required? Would `bionic` do? In bionic, clang 7 is available from distro.
* Which Python version needs to be preinstalled? Tools/ssl/multissltests.py says it's supposed to be compatible with 2.7 and 3.4-3.7. Options are, by decreasing "sum of flexibility+reliability":
  * use 3.5.1 or 2.7.12 from distro
  * hard require `language:python; python: 3.7` (then we cannot use custom `clang`)
  * use another pyenv-preinstalled version (there are also 3.6 and 2.7 that are supposed to be preinstalled but there might be none)

> New changeset 94d2c8df1a7657015a2fcdb4c4d43392f91f8348 by Inada Naoki in branch 'master':
bpo-39035: travis: Don't use beta group (GH-17602)

> INADA-san: do you want to close this issue since you pushed changes?

This may actually fix the problem.

`group: beta` is undocumented. It was required long ago once to try out a new feature but I've absolutely no idea what it does (and if it does anything) now. It's quite possible that it can give us some beta unsupported VM image.

While we are at it, shall we enable build config validation (https://docs.travis-ci.com/user/build-config-validation)?

It would produce warnings for outdated keys like this.

New changeset 7699281b72b862797a89fcad004da8f58e93c800 by Ned Deily (Inada Naoki) in branch '3.6':
bpo-39035: travis: Update image to xenial (GH-17622)
https://github.com/python/cpython/commit/7699281b72b862797a89fcad004da8f58e93c800

New changeset 9af497419540cdb4659927e66c67d861c5ea48c2 by Ned Deily (Inada Naoki) in branch '3.7':
bpo-39035: travis: Don't use beta group (GH-17604)
https://github.com/python/cpython/commit/9af497419540cdb4659927e66c67d861c5ea48c2

I didn't notice the error in the last days, so I close the issue.

I would be nice to merge PR 17623, but that can be done later (by Larry Hastings, the 3.5 release manager).