Drop python2 support

[thomas-riccardi]

Python 2 will become unmaintained very soon (https://www.python.org/doc/sunset-python-2/).

Moving to python3-only will give us not only faster tests but also new useful features (see #79 for example).

We could maintain an old branch for python2 related to an old branch for deepocli if some clients are really stuck with python2, but we should at least start moving toward python3 only.

[thomas-riccardi]

We could also use raise ... from: https://stackoverflow.com/a/792163

... after some tests __cause__ doesn't survive billiard though..

[maingoh]

We should also remove all versions < 3.6 and add the new ones in the dmake for testing

[thomas-riccardi]

bump
It seems to block some cleaning in deepomatic-workflows, cf https://github.com/Deepomatic/deepomatic-workflows/pull/282#discussion_r614716386