lookup_channel: don't use target_field to get cast function

[Hyask]

At least in Django 1.8.18, target_field does not exists for OneToOneField. Using the field attribute of the relation fixes the exception and makes my forms to work as expected, but I'm not able to test on other Django versions.
Also, I'm not sure if it does not break this #153 again. Could someone check that?

[crucialfelix]

You can run tox and it will check all combinations of python and django versions. Just install the requirements-test.txt and run make test

[Hyask]

Is it not what your CI setup does? Even though, is the specific case of the issue covered?

[crucialfelix]

Yes, that's what the CI does.

What I'm saying is that you can easily run it yourself to test before making a PR. You said "I'm not able to test on other Django versions", but it's actually quite trivial to do it. just make test

It did pass the tests, so it probably works. I would have to make sure that part of the code has coverage.

[Hyask]

Okay, I see your point. I've added a section in your README to explain that part.

[crucialfelix]

I added tests and checked that your fix worked across all versions. Unfortunately I didn't cherry pick your fix, but I did give you credit in the commit message. The fix is now released, Thanks !

[Hyask]

That's fine, thanks! :)