py3: use the train test template and other updatestrain-em2.3.0

Use the unified unit test Zuul template for train, according the
community goal. Also, update the setup.cfg classifiers accordingly.

Update the debug test environment. Adding "debug-py36" is enough;
if a distribution supports 3.7, the "debug" tox environment
can be used.

Finally, keep only the most recent environment in the default
set executed when calling tox.

Story: #2005924
Task: #34244

Change-Id: Iad4a6e933ad416eb67a2f144540fe79e97938bec

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