Dj-stripe: Fix system check failure on Django 3.1: (fields.W903) NullBooleanField is deprecated.

[therefromhere]

eg from https://travis-ci.org/github/dj-stripe/dj-stripe/jobs/693027232

E           djstripe.Account.livemode: (fields.W903) NullBooleanField is deprecated. Support for it (except in historical migrations) will be removed in Django 4.0.
E               HINT: Use BooleanField(null=True) instead.
E           djstripe.ApplicationFee.livemode: (fields.W903) NullBooleanField is deprecated. Support for it (except in historical migrations) will be removed in Django 4.0.
E               HINT: Use BooleanField(null=True) instead.
E           djstripe.ApplicationFeeRefund.livemode: (fields.W903) NullBooleanField is deprecated. Support for it (except in historical migrations) will be removed in Django 4.0.
E               HINT: Use BooleanField(null=True) instead.

Currently muted as part of #1173, but we should fix properly.

[trey]

Just curious if we know when this will be officially released? Or is it a good idea to just run off the master branch?

[anuzis]

Any update?

[trey]

@anuzis, I think this may be a good place to keep up with what's going on: https://github.com/dj-stripe/dj-stripe/issues/1265

[anuzis]

@trey Perfect! Thanks.

[jleclanche]

@trey @anuzis 2.4.0 is planned for tonight / tomorrow.

[trey]

@jleclanche Thank you! 😃