Removed dependence on six library

[al42and]

Currently, picklefield requires six for singe one-line function with_metaclass.

I'm not sure whether such change is desirable, but I think removing nearly-unnecessary dependency might be convenient.

[budlight]

I'm sure the reason he requires six is to eventually make it python 2 and 3 compatible. This sounds like going backwards.

[rafales]

@budlight it is python 3 compatible. If you encountered any bugs while using picklefield with python 3 then please fill a bug report.

[budlight]

@rafales sorry you thought I was saying it wasn't python 3 compatible, but it wasn't my place to flat out say it is, because I had never tried it.

[tacaswell]

It looks like the current version of django ships with it's own copy of six, if any change should be made it is to import from django's version from django.utils import six.

six is more-or-less a must-have library anyway so I don't see the benefit of this.

[charettes]

I agree with @tacaswell that we should import six from django.utils instead. Will submit a PR with the required changes.