Update docs to maintained djangorestframework-guardian2

[johnthagen]

Description

djangorestframework-guardian has not had a new release for over 3 years

• https://pypi.org/project/djangorestframework-guardian/#history

There are no commits for 2.5 years:

• https://github.com/rpkilby/django-rest-framework-guardian/commits/master

The maintainer has not responded to an issue about this for over a year

• Is this project still maintained? rpkilby/django-rest-framework-guardian#19

djangorestframework-guardian2 is a friendly fork I created that supports the currently supported Django and Python versions

• https://pypi.org/project/djangorestframework-guardian2/

Out of courtesy (not trying to bug the maintainer more), I will CC @rpkilby here in case he would like to contend this (in which case I'll happily allow him to maintain the original package again if he desires).

[johnthagen]

CI failure looks to be unrelated:

DeprecationWarning: Running attrs on Python 3.6 is deprecated & we intend to drop support soon. If that's a problem for you, please let us know why & we MAY re-evaluate: <https://github.com/python-attrs/attrs/pull/993>

• Drop 3.6 & switch to 100% static packaging configuration python-attrs/attrs#993

[tomchristie]

Thanks @johnthagen!

CI failure looks to be unrelated

Hrm, yup. I'm not sure where it's being pulled in from but looks like we need to pin to an older version of attrs.
(For the test suite, not as a dependancy)

[tomchristie]

@johnthagen The build failures ought to be addressed if you merge master now.

[johnthagen]

@tomchristie Thanks, done.