migrate tests from nose to pytest

I'd like to consider the idea of migrating the test set of all the python modules to pytest

It would help for several aspects, especially via the test fixtures mechanism, like make it easier to write tests for things like celery tasks (see [celery fixtures](http://docs.celeryproject.org/en/latest/userguide /testing.html#fixture) for example)

The main problem is that it will most probably require to migrate all tests at once, which seems quite a job.

Update: we've decided to go ahead with this

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Migrated from T1261 (view on Phabricator)