Minor refactoring for Python 3 compatibility

[hvelarde]

No description provided.

[hvelarde]

@plone/framework-team please review.

[hvelarde]

this seems to be zope.interface fault:

https://github.com/zopefoundation/zope.interface/blob/4.5.0/src/zope/interface/common/sequence.py#L34-L38

we need to remove __getitem__ from there also.

[jensens]

cc @pbauer

[gforcada]

@pbauer as you are more into the python3... I was fixing a test failure in plone.app.querystring that lead me to the commit I made here in this branch.

With my commit and without @hvelarde commit both p.a.querystring and p.a.contentlisting tests passes (circa one test that fails anyway in p.a.contentlisting). With @hvelarde and my commits there are 5 test failures.

@hvelarde which tests where you trying to fix? 🤔

[hvelarde]

@gforcada I'm using a different approach to achieve Python 3 compatibility (using pylint --py3k to find issues in our code); as mentioned in the code itself __getslice__ has been deprecated since Python 2.0 and we need to remove it.