Validates that related_name is a valid Python id or ends with a '+' and
it's not a keyword. Without a check it passed silently leading to
unpredictable problems.

Thanks Konrad Świat for the initial work.

Enabling the attribute causes get_object() to perform its lookup
using both the primary key and the slug.

Thanks tchaumeny and Loic for reviews.

Thanks Claude Paroz, Riccardo Magliocchetti, and Damian Moore
for contributions to the patch.

Forward port of e9c8aefb from stable/1.7.x

Thanks oinopion for the suggestion and jarus for the initial patch.

Thanks Collin Anderson for the review.

This ensures consistency between Python 3.4+ and earlier versions.

Thanks Harry Percival for the report.

Thanks Claude Paroz for partial fix and Simon Charrette for review

Thanks maxi for the report, Shai Berger for his help with the patch
and Tim Graham for the review.

Thanks Tim Graham for the review.

Thanks "djbug" for the report.

Thanks lidaobing for the suggestion.

Thanks Shai Berger for review.

Refactored old tests that were rewritten 1:1 from doctests.

Thanks Claude Paroz for suggestions and review.

Thanks sky-chen for the report.