  
          if not isinstance(field, (models.ForeignKey, models.ManyToManyField)):
              return [
                  checks.Error(
                      "CurrentSiteManager cannot use '%s.%s' as it is not a ForeignKey or ManyToManyField." % (
                          self.model._meta.object_name, field_name
                      ),
                      obj=self,
                      id='sites.E002',
                  )
 
</s> remove         if not isinstance(field, (models.ForeignKey, models.ManyToManyField)):
 </s> add         if not field.many_to_many and not isinstance(field, (models.ForeignKey)): </s> remove                 "not a ForeignKey or ManyToManyField.",
 </s> add                 "not a foreign key or a many-to-many field.", </s> remove             if not isinstance(field, (models.ForeignKey, models.ManyToManyField)):
                return must_be('a ForeignKey or ManyToManyField',
 </s> add             if not field.many_to_many and not isinstance(field, models.ForeignKey):
                return must_be('a foreign key or a many-to-many field', </s> remove   ``ForeignKey`` or ``ManyToManyField``.
 </s> add   foreign key or a many-to-many field. </s> remove                         "The value of '%s' must not be a ManyToManyField." % label,
 </s> add                         "The value of '%s' must not be a many-to-many field." % label,
                    "CurrentSiteManager cannot use '%s.%s' as it is not a foreign key or a many-to-many field." % (
  
          if not isinstance(field, (models.ForeignKey, models.ManyToManyField)):
              return [
                  checks.Error(
                    "CurrentSiteManager cannot use '%s.%s' as it is not a foreign key or a many-to-many field." % (
                          self.model._meta.object_name, field_name
                      ),
                      obj=self,
                      id='sites.E002',
                  )
keep keep keep keep replace keep keep keep keep keep
 Refs #24227 -- Replaced M2M isinstance checks by field.many_to_many

Thanks Markus Holtermann, Collin Anderson and Tim Graham for the reviews. 
django/contrib/sites/managers.py