Don't specify version in Tox

[adriangonz]

Fixes #1247

Changelog

• Run tox with whatever Python version is present

[seldondev]

Thu Feb 13 10:35:23 UTC 2020
The logs for [pr-build] [1] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-1433/1.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-1433 --build=1

[seldondev]

Thu Feb 13 10:35:30 UTC 2020
The logs for [lint] [2] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-1433/2.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-1433 --build=2

[adriangonz]

/cc @cliveseldon @RafalSkolasinski

@cliveseldon this should fix the Python tests whenever py37 is not present (e.g. because py36 is installed).

@RafalSkolasinski to run the tests across multiple versions we could add more versions to the envlist in the setup.cfg file and then install them as well on the seldonio/python-builder image. We could explore this for #1357.

[ukclivecox]

/approve

[seldondev]

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