Fix 'plone.belowcontentbody.relateditems' viewlet

Avoid trying to display items if the user has no permission to view them (like content in Private state)
Update package dependencies

Clearly specify this branch is for Plone >=4.3 only (in fact, should be 5.0).
Remove dependency on unittest2

We are not going to test against Python 2.6 anymore on Plone 5.0; use test discovery also.
Merge remote-tracking branch 'origin/master' into plip-13770-portal-tools-removal