Add Django 2.2 and 3.0 Support

[studybuffalo]

Major changes to allow Django 3.0 support

• Switched import of python_2_unicode_compatible to the six library, as it is removed from Django.
• Switched patch_logger (from Django) tests to assertLogs (from unittest), as patch_logger functionality is being dropped by Django.
• Created a temporary test mixin that enables support of assertLogs in Python 2.7; once Python 2.7 support is dropped this mixin can be removed and all tests should continue to pass.
• Pinned sorl-thumbnail to a specific commit that supports Django 1.11 to Django 3.0 - intent would be for this to be returned back to a released version as soon as the next release is made.

Other minor changes

• Updated .travis.yml to remove unused statements and add recommended statements.

Relevant Issues: #291 #295 #296 #299

[studybuffalo]

Question to be resolved: is it acceptable to continue with sorl-thumbnail pinned to the specific commit, or do we remove it as dependency as per issue #297

[coveralls]

Coverage remained the same at 88.635% when pulling 4c52bb2 on studybuffalo:support-django-22-and-30 into 29f8208 on dokterbob:master.

[coveralls]

Coverage remained the same at 88.635% when pulling 5bafbdb on studybuffalo:support-django-22-and-30 into 29f8208 on dokterbob:master.

[dokterbob]

Great work! I'll try and review it ASAP.

Question to be resolved: is it acceptable to continue with sorl-thumbnail pinned to the specific commit, or do we remove it as dependency as per issue #297
I think pinning the version is good enough for now. There might be a future release of sorl-thumbnail - but removing the dependency is potentially a lot of work and until it's done this will allow users to stay using django-newsletter. :)

Thanks again!

[dokterbob]

Wow! Such a clean contribution! I've already reviewed it, there's only one comment, really:

Please add comments in the Travis file as well as in the requirements.txt about the pinned version of sorl-thumbnail, so that we're reminded to remove it once a new release comes out. I was also wondering why the dependency needs to be explicitly installed with Travis, while it's already in the requirements. That suggests it might create problems for users and/or require additional install instructions. Could you please verify whether it is necessary (to add this install step in Travis)?

Lastly, once the patch has been reviewed, I would appreciate if you could do a pipenv install -r requirements.txt to update the Pipenv file as well as the lockfile.

[studybuffalo]

Thanks for catching that - I learned that requirements.txt editable dependencies don't work the same way for setuptools. I put the Travis file changes in to get things moving forward for development and then forgot about it.

I think I have got a simple enough fix working now that resolves those noted issues and we can easily pull it out once a new sorl-thumbnail version is released. I also made sure to flag it all with TODO comments.

With these changes passing CI, I have also gone and updated the lockfile as requested.

[dokterbob]

I would request you find a way not to hardcode the line number in setup.py, please see comment.