ci: Travis Job for Python 3.8 on Linux does not actually run Python 3.8?

[EricCousineau-TRI]

Issue description

If you look at any job Python 3.8, c++17, gcc 7 (w/o numpy/scipy) in Travis CI builds, they don't seem to actually be using Python 3.8. (Not sure if this is due to a patch I submitted...)

Came across this is in our fork, which effectively uses the same CI scripts here.

Reproducible example code

See this build:

https://travis-ci.org/github/pybind/pybind11/jobs/640390164#L467

-- Found PythonInterp: /opt/pyenv/shims/python3 (found version "3.6.7")

[carlsonmark]

Do you think that changing PY_CMD=python3 to PY_CMD=python3.8 would be sufficient to fix that? Or is there more todo?

[EricCousineau-TRI]

Yup, I think that would be it!

Testing it out in my fork, can PR the patch if I can confirm it works:
RobotLocomotion#40

(I may also disable one of the PyPy jobs, as that seems to be freezing on a download?)

[carlsonmark]

OK. I think if that doesn't work, then maybe some clever use of update-alternatives may help. I'm not really familiar with any of that though. I'm just starting to look into travis-ci myself, here.

Good luck!

[carlsonmark]

Looks like you will need to add this package too:

 - python3.8-distutils

Which allows python3.8 -m pip to work on xenial.

[carlsonmark]

My mistake, I had installed python3.8 from snap a while ago and uninstalled it, so my environment is not clean.

This may be the way to install pip in python3.8 (on xenial) using deadsnakes: deadsnakes/issues#95

curl https://bootstrap.pypa.io/get-pip.py | python3.8

But you might want to read that page first in case you think it may be a bad idea.

[EricCousineau-TRI]

Hm... Lemme try out python3 -m ensurepip, as that tends to work when build CPython from source. Thanks!

[chaitan94]

Possibly related: #1090

[henryiii]

This has all been wiped out and replaced by GHA, and those do run the correct versions. :)