[QA] Changes to CI for Python jobs

[Kunal Mehta]

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Hi,

There are a few changes to the Python `tox` jobs that CI is running:
1. Most importantly, Python 3.5 is now the default version when
invoked with `tox`. This can be overridden by using basepython[1].
2. Multiple Python 3 versions are available: 3.4, 3.5, 3.6, and 3.7
(T191764).
3. It's now based on Debian Stretch, instead of Jessie, so some system
libraries are newer.

It's possible that #1 will break some repos, but with the Python 2 EOL
coming very soon, it was time to just do it.

You can take advantage of #2 by having multiple environments in
tox.ini, see keyholder[2] for an example.

I'm still working on rolling these out, so it might take another day
or two for it to take effect. If you run into any issues/problems,
please file a bug in the Continuous-Integration-Config Phabricator
project.

[1]
https://tox.readthedocs.io/en/latest/example/general.html#basepython-def
aults-overriding
[2]
https://gerrit.wikimedia.org/r/plugins/gitiles/operations/software/keyho
lder/+/master/tox.ini#3

- -- Legoktm
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