Support Django up to 1.11, fix DeprecationWarnings, fixes #89

[fdintino]

I've also set warnings to raise as errors in unit tests to prevent DeprecationWarnings from being reintroduced accidentally in future.

I've added the intermediate django versions to tox.ini and to the travis build matrix. I've also added py36-master to the build matrix, though marked as an allowed failure so that it can fail without affecting whether the build as a whole passes. I've made it so that the tests pass against the django master branch (2.0), so it currently passes on travis, but since it's under development it's possible that there could be an upstream change that might breaks things.

(As an aside, keeping django support going back to 1.5 is laudable, but I don't think anyone would hold it against you if you only supported Django >= 1.8).

[nemesifier]

This pull request is a big clean up of the codebase.

I hope this can be merged soon. I would also love to implement the solution I just suggested in #102 (comment).

I also agree supporting django >= 1.8 would be enough for the current use case.

[jharrell]

Any timeline on this? Just tried updating a Django project from 1.8 to 1.10 and saw an extremely large increase in deprecation warnings (mostly from this package.)

I'm also happy to submit a smaller pull request directly addressing my issue

Also agree that at this point, supporting >=1.8 is a pretty reasonable choice, but I also think keeping backwards compatibility is an awesome feature of this package.

[nemesifier]

@gregmuellegger I also would like to see this merged. Let us know!

[foosinn]

@gregmuellegger I would like to see it merged, too.

[ahsan-ul-haq]

@gregmuellegger any updates on when it will get merge?

[schenedx]

@gregmuellegger
I am an engineer working for edX. We are looking to upgrade one of our service to Django 1.11. That service uses this library.
We really hope this PR can be merged so it's no longer considered an upgrade road block.
The Plan B for us is to fork this PR within our private organization github repo and merge the changes on this PR in.
Please let us know when this PR can be merged. Thank you!

[bcail]

@gregmuellegger Thanks for your work on this library! We use it as well, and need it to work with django 1.11. Is there anyone else who has commit privileges on this repo, or anyone you could add? That would let these changes move forward, and could make the burden lighter for you.

[nemesifier]

@gregmuellegger I'm also willing to help out since openwisp2 uses this django app as a dependency.

[KKomarov]

As workaround, you can do pip install https://github.com/theatlantic/django-sortedm2m/tarball/pr/django-1.11-support but not for prod :)

[gregmuellegger]

1.4.0 is out containing these changes! https://pypi.python.org/pypi/django-sortedm2m

Sorry for the very long wait! I'm super super occupied currently in my private live which sets open source a little on the back burner currently. I will try to find a better maintaining solution for the long run.

For now, thank you everyone for being so patient and very constructive in your comments and contributions!

I'm would appreciate any positive feedback about if sortedm2m now works for you as expected with Django 1.11.

[swizzlevixen]

With the caveat that I'm still in the midst of (belatedly) writing unit tests for my Django apps: from a manual exercise of the website and checking the warning logs, this got rid of all of the sortedm2m-related deprecation warnings, and everything seems to be operating properly.