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MST-34
After executing python-modernize and isort, few changes occurred in
import sequence as well as unicode func is replaced by six.text_type
func.
INCR-478
Django 2.0 will make this field required for `ForeignKey` and `OneToOneFields`.
In previous versions the option defaulted to `models.CASCADE` when not
specified. This change should make the deprecation warnings in the current
Django version go away.
The migrations where also modified, but the changes should not cause a change in
the database schema since `models.CASCADE` was already the old default.
