Bugzilla

Ah so it turns out that Celery doesn't yet support Python 3.7 (fix on celery master, not yet released: https://github.com/celery/celery/issues/4500), so we'll need to use Python 3.6 instead for now. Once they support Python 3.7, and we've had a chance to figure out the celery upgrade issue in bug 1337717, we can then upgrade. This incompatibility didn't show up before now likely due to bug 1349362.

Ah so it turns out that Celery doesn't yet support Python 3.7 (fix on celery master, not yet released: https://github.com/celery/celery/issues/4500), so we'll need to use Python 3.6 instead for now. Once they support Python 3.7, and we've had a chance to figure out the celery upgrade issue in bug 1337717, we can then upgrade. 

This incompatibility didn't show up during tests likely due to bug 1349362. It could be seen via Papertrail in the form of:

```
app/worker_store_pulse_jobs.4:   File "/app/.heroku/python/lib/python3.7/site-packages/celery/concurrency/prefork.py", line 20, in <module> 
app/worker_store_pulse_jobs.4:     from celery.concurrency.base import BasePool 
app/worker_store_pulse_jobs.4:   File "/app/.heroku/python/lib/python3.7/site-packages/celery/concurrency/base.py", line 21, in <module> 
app/worker_store_pulse_jobs.4:     from celery.utils import timer2 
app/worker_store_pulse_jobs.4:   File "/app/.heroku/python/lib/python3.7/site-packages/celery/utils/timer2.py", line 19 
app/worker_store_pulse_jobs.4:     from kombu.async.timer import Entry, Timer as Schedule, to_timestamp, logger 
app/worker_store_pulse_jobs.4:                    ^ 
app/worker_store_pulse_jobs.4: SyntaxError: invalid syntax
```