Display Python and Celery info and deprecation warnings on startup.
Display Python and Celery info and deprecation warnings on startup.

When starting the worker, we now check the Python version. If it's
Python 2, we warn that Python 2 and Celery 3 are no longer supported and
may have security risks, recommending an upgrade to Python 3.

If running on Python 3, we show the Python and Celery versions (as a
reference) and have a general recommendation to keep up with versions.

Testing Done:
Tested running under Python 2 and 3, checking the resulting output
and version numbers.

Reviewed at https://reviews.reviewboard.org/r/12158/

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