Update tests

[MaferMazu]

I was testing some PR that you have for jsonfield

#262
This fixes the warnings and the name of a file that causes the circle ci test to fail.

#261
And this one adds test for django 3.1 and 3.2

I think it would be nice to add them to the main branch. I tried both PR together and work fine (https://app.circleci.com/pipelines/github/MaferMazu/jsonfield?branch=mfmz%2Ffix-ci-and-test-django3.2)

Hopefully you can add them so that the library remains updated.

Any doubt or advance I am at your service.

Thanks for your consideration.