Fix compatibility with django-rest-framework 3.14.0 having depreciated NullBooleanField

[JoelLefkowitz]

As pointed out in #810, django-rest-framework 3.14.0 has depreciated its NullBooleanField.

For the sake of backwards compatibility we can drop the NullBooleanField types when the installed version of django-rest-framework exceeds 3.14.0.

[JoelLefkowitz]

Before merging I would like someone to have a look at this PR for a soundness check. Once merged I'll deploy a new release to pypi.

[yamac-kurtulus]

Just a heads up, models.NullBooleanField is still there, serializer field is deprecated. Though it is completely understandable if you want to remove that too.

[JorisBenschop]

indeed, at this moment when a user installs the latest drf and drf-yasg, there is a crash in "serializer_field_to_basic_type" (field.py).
I should note that the error the user gets is really cryptic. Because the original error is caught, it is replaced by a "module does not define a %s attribute/class", which is not the error at all, and puts people on a wild goose chase