Django 3.0 compatibility about django-treewidget

[jerch]

@jphilip Well I have not tested it myself with Django 2.2 or 3.0 yet. Does it still work there beside the six issue? Note that there are several possible breaking points that need to be tested:

• Does the field construction logic still work? Imho this did not change in Django with 2.2/3.0. Affected files: fields.py, maybe template files.
• Does it still integrate well with newer admin stylesheets? Not sure Django changed them, possibly affected default.css.
• Does it still work with updated tree model packages? The unified TreeQuerySet only implements treebeard and mptt hooks, affected file tree.py.
• Is the JS part still compatible to newer jQuery version delivered with newer Django versions? Is the workaround still needed, does it still work as described?
• Optional: jstree could use an update as well. Atm 3.3.4 is delivered, official repo lists 3.3.9 as current release. Affected files: default.js, formatters.py.

For the six part imho we should drop Python 2 and Django 1.11 support, both are beyond their official support cycle now.

If you want to tackle some issues listed above - sure thing, would be nice to see it being updated. If you have a good idea, how this package could be tested automatically without setting up expensive selenium tests - I am all ears. For the listed versions I did it manually in the past, which gets exhausting for multiple versions pretty soon.