Is necromancy just speaking to the dead, as per Candy's definition?
Merriam Webster says 1 : conjuration of the spirits of the dead for purposes of magically revealing the future or influencing the course of events
Dictionary.com says 1.a method of divination through alleged communication with the dead; black art.
Catholic Encyclopedia says "a special mode of divination by the evocation of the dead.
Clearly there is more to necromancy than just speaking to the dead. How many times do people go to a cemetery and speak to someone at their grave site? Is that necromancy?
Necromancy would include summoning or communicating with the dead for manipulation in some way, usually for information. As a Biblical example, in 1 Samuel 28, Saul has a witch summon the soul of Samuel in order to get information on how to win the war with the Philistines.
Any sort of occult activity is prohibited by the Catholic Church:
CCC #2116 All forms of divination are to be rejected: recourse to Satan or demons, conjuring up the dead or other practices falsely supposed to "unveil" the future.48 Consulting horoscopes, astrology, palm reading, interpretation of omens and lots, the phenomena of clairvoyance, and recourse to mediums all conceal a desire for power over time, history, and, in the last analysis, other human beings, as well as a wish to conciliate hidden powers. They contradict the honor, respect, and loving fear that we owe to God alone.
2117 All practices of magic or sorcery, by which one attempts to tame occult powers, so as to place them at one's service and have a supernatural power over others - even if this were for the sake of restoring their health - are gravely contrary to the virtue of religion. These practices are even more to be condemned when accompanied by the intention of harming someone, or when they have recourse to the intervention of demons. Wearing charms is also reprehensible. Spiritism often implies divination or magical practices; the Church for her part warns the faithful against it. Recourse to so-called traditional cures does not justify either the invocation of evil powers or the exploitation of another's credulity.
When Catholics ask for the prayers of Mary, or any other saint, we are not expecting them to appear before us. We are not asking for information. We are simply asking them to join their prayers to ours, in the same way that we ask our friends and family to pray for us.
Yes, but they're dead, you might reply. But, Luke 20:38 says "For he is not a God of the dead, but of the living: for all live unto him."
Well, they aren't aware of us in heaven, you insist. However, Hebrews 12:1 says that "we also are compassed about with so great a cloud of witnesses."
Additional reading at Catholic Answers and Scripture Catholic.