Remove usage of depreciated NullBooleanField for drf 3.14.0

[sarahboyce]

There is a new drf release https://www.django-rest-framework.org/community/release-notes/#3140
Here NullBooleanField is no longer available and so I removed the one reference to allow support for drf 3.14.0

It was also not required as the isinstance(field, BooleanField) would return true for a NullBooleanFIeld due to object inheritance:

>>> from rest_framework.fields import NullBooleanField
>>> field = NullBooleanField()
>>> from rest_framework.fields import BooleanField
>>> isinstance(field, BooleanField)
True
>>> 

So I think we should be good 👍

Thank you for the package ❤️ @FlipperPA

[FlipperPA]

Thank you, @sarahboyce - I will take a look at this today.

[sarahboyce]

Thank you @FlipperPA 🎉
Let me know if you need anything from me 👍

[FlipperPA]

Thank you, @sarahboyce!

[FlipperPA]

Thank you! This has been published to PyPI at 2.2.0:

https://github.com/wharton/drf-excel/releases/tag/2.2.0

https://pypi.org/project/drf-excel/2.2.0/