'Deletes on concurrent transactions don\'t collide and lock the database. Regression for #9479'

@skipUnlessDBFeature('test_db_allows_multiple_connections')
def test_concurrent_delete(self):

    b1 = Book(id=1, pagecount=100)
    b2 = Book(id=2, pagecount=200)
    b3 = Book(id=3, pagecount=300)
    b1.save()
    b2.save()
    b3.save()
    transaction.commit()
    self.assertEqual(3, Book.objects.count())
    cursor2 = self.conn2.cursor()
    cursor2.execute('DELETE    from    delete_regress_book    WHERE    id=1')
    self.conn2._commit()
    Book.objects.filter(pagecount__lt=250).delete()
    transaction.commit()
    self.assertEqual(1, Book.objects.count())
    transaction.commit()
