2.2.0: Use Redis for locking

Replace django-db-locking with Redis locks. This removes a dependency on
the former, but adds one on the latter.

python 3.7 compatibility

2.1.21

version bump to 2.1.21

Add waffle flag to disable sms processing

In case you want to temporarily disable switch processing, we now allow for a waffle switch to control the queue.

Update six requirement version

Update six requirement version

update requests to latest version

Update requests to latest version

Format numbers to 'msisdn' format

• the TSL api needs phonenumbers without leading '+' and leading 0.
  Formatting of phone number adapted to return msisdn like numbers

• update celery requirements

upgrade to django 2.1.5

This release has not and will not be tested for django 1.x.

Django requirement not pinned

Don't pin the django requirement to specific 1.x version anymore. We use the gateway mostly as a dependency in other django projects -- which use django > 2.0.

Ideally, we'd make the gateway fully django 2.0 compatible (or throw it out altogether).

Pip 10 compatibility

Warning: Not backwards compatible with older pip versions !

Django 2.0 compatibility

Adds support for django 2.0