RemovedInDjango41Warning for default_app_config

[willkg]

dockerflow kicks up a RemovedInDjango41Warning because there's a default_app_config defined in dockerflow.django.__init__.py.

As of 3.2, Django automatically finds AppConfig things:

https://docs.djangoproject.com/en/3.2/releases/3.2/#automatic-appconfig-discovery

Further, as of 3.2 setting default_app_config is deprecated:

https://docs.djangoproject.com/en/3.2/releases/3.2/#id3

[jwhitlock]

Fixed in PR #67