Necromancy is asking the dead for advice. Nothing about asking for prayers is asking for advice.
The Bible doesn't make such a distinction. We are forbidden from contacting the dead. Period.
Having debated this issue with Catholics many times, here's how you're going to respond:
First, you'll say that the dead aren't really dead. In which case I will respond to you by asking, who are the people the Bible calls "the dead in Christ". You, then, will ignore my question, and cite Luke 20:38, proclaiming that God is not the God of the dead, but of the living, while completely ignoring Romans 14:9 that says that Christ is Lord of both the living and the dead.
You will then ignore that verse and go on to remind me about the Mount of Transfiguration in Matthew 17, in which Jesus takes Peter, James, and John up to a high mountain where He spoke with Moses and Elijah. I will, of course, remind you that (a) only Jesus spoke to Moses and Elijah, (b) that Jesus did so while in a transfigured state, in which, more than any time during His incarnation, He manifested His divine state, and (c) that Jesus Christ, in His authority as God, as the Lord of both the living and dead, has privileges and authorities that are not extended to man.
After that, of course, you'll start grasping straws and move on to Hebrews 12, and the "great cloud of witnesses", upon which time I will remind you that this does not refer to a literal grandstand full of disembodied spirits, but is a metaphor, referring to the testimonies of those saints (and, by "saints", I mean saints in the Biblical sense, not saints in the Roman Catholic "do a couple of card tricks from beyond the grave and your in" sense) who have gone before and who's lives serve to be an encouragement and a testimony to us, and that nowhere in the passage do we see anyone attempting to contact or to pray to them.
Finally, you'll tell me about Revelation 5, in which we're told that the elders hold bowls of insense which are the prayers of the saints, and I'll remind you that the passage says nothing about our praying to them or their being able to hear us.
Does that about sum it up? Would you like to move on to Purgatory next and tell me how the purpose of Purgatory isn't really to expiate sin?