The support matrix between versions of Celery, Django, and Review Board

is tricky and complex. Celery 3.x, the version we were on, doesn't

support running on Python 3. Celery 4.x, which does support Python 3,

doesn't support Django 1.6.
This change makes us conditionally use Celery 4.x when running on Python

3, and Celery 3.x when on Python 2.7. The 3.x version is pinned to

3.1.25 or newer, which introduces a forward-compatible wire protocol

with 4.x. We therefore end up with this support matrix:
RB    | Python | Django | Celery

------+--------+--------+------------

3.0.x | 2.7    | 1.6    | 3.1.25+, <4

4.0.x | 2.7    | 1.11   | 3.1.25+, <4

4.0.x | 3.6+   | 1.11   | 4.4+, <5

Configured and ran Review Bot with the server and worker running on

both Python 2.7 and 3.8. Saw that it worked with both the 3.1.25 and

4.4.7 versions of Celery.