url() is deprecated in Django 3.1

[smithdc1]

CI is raising warnings as url() is deprecated in the upcoming release of Django.

https://travis-ci.org/github/encode/django-rest-framework/jobs/707134589

[auvipy]

check #7325

[sanjusci]

Please see:
#7420

[adamchainz]

I re-did #7420 in #7512, with some fixes (see commits).