Fix o2o relation

[robertpro]

This PR fixes: #662

I tested manually like this:

def get_reverse_fields(model, local_field_names):
    for name, attr in model.__dict__.items():
        # Don't duplicate any local fields
        if name in local_field_names:
            continue
        r = getattr(attr, "rel", None)
        r2 = getattr(attr, "related", None)

        if r is not None or r2 is not None:
            print(r, r2)

        related = getattr(attr, "rel", None)
        if isinstance(related, models.ManyToOneRel):
            yield (name, related)
        elif isinstance(related, models.ManyToManyRel) and not related.symmetrical:
            yield (name, related)

And the result (on runserver output):

<ManyToOneRel: event.companion> None
None <OneToOneRel: event.invitation>

Which means O2O relation is still using "related"

[coveralls]

Coverage remained the same at 92.718% when pulling 94602c7 on robertpro:fix-o2o-relation into bad8e13 on graphql-python:master.

[mvanlonden]

Thank you!