* Upgrade Django to 3.0 and other dependencies to recent versions
* Fix otp version contsraint
* Remove six dependency
* Resolve some warnings
* Fix failing tests
* Update django-countries
* Resolve all RemovedInDjango31Warnings in test suite
* Run isort
* Fix import
* Update PostgreSQL version on travis
If the provided mail address has not ordered anything, there will still
be a mail generated and sent to an invalid mail address, to avoid
obvious timing attacks to determine active users.