on_conflict/upsert does not allow `pk` as a valid conflict target

[Photonios]

class MyModel(psqlextra.Model):
     title = models.CharField(max_length=14)

MyMode.objects.on_conflict(['pk'], ConflictAction.UPDATE).insert(pk=0, title='great')

Raises the following exception:

django.core.exceptions.SuspiciousOperation: pk is not a valid conflict target, specify a list of column names, or tuples with column names and hstore key.

In Django, pk is a special keyword that always refers to the primary key of a model. psqlextra/compiler.py should also look at model._meta.pk when validating conflict targets.