prefetch_terms(queryset) evaluates queryset twice via values_list()

[jacobtylerwalls]

prefetch_terms(queryset) evaluates the queryset once here:

django-guardian/guardian/core.py

Line 21 in 77de203

pks = [force_str(pk) for pk in objects.values_list('pk', flat=True)]

...and again here:

django-guardian/guardian/core.py

Line 250 in 77de203

for obj in objects:

Planning to submit a patch that removes the values_list() for a simple iteration over the queryset.