Is practicing witchcraft a sin?

Yes. Witchcraft is any supernatural effect where God is not the source of it. This implies Satan is the source of it, since we cannot perform supernatural affects without external help.

What the Bible says about it:

Deut 18:10 — ~"There shall not be found among you anyone who makes his son or his daughter pass through the fire, one who uses divination, one who practices witchcraft, or one who interprets omens, or a sorcerer,

Deut 18:14 — ~"For those nations, which you shall dispossess, listen to those who practice witchcraft and to diviners, but as for you, the LORD your God has not allowed you {to do} so.

2 Kings 17:17 — Then they made their sons and their daughters pass through the fire, and practiced divination and enchantments, and sold themselves to do evil in the sight of the LORD, provoking Him.

2 Kings 21:6 — He made his son pass through the fire, practiced witchcraft and used divination, and dealt with mediums and spiritists. He did much evil in the sight of the LORD provoking {Him to anger.}

2 Chron 33:6 — He made his sons pass through the fire in the valley of Ben-hinnom; and he practiced witchcraft, used divination, practiced sorcery and dealt with mediums and spiritists. He did much evil in the sight of the LORD, provoking Him {to anger.}

Nahum 3:4 — {All} because of the many harlotries of the harlot, The charming one, the mistress of sorceries, Who sells nations by her harlotries And families by her sorceries.

Acts 8:9 — Now there was a man named Simon, who formerly was practicing magic in the city and astonishing the people of Samaria, claiming to be someone great;

Acts 19:19 — And many of those who practiced magic brought their books together and {began} burning them in the sight of everyone; and they counted up the price of them and found it fifty thousand pieces of silver.

Gal 5:19-20 — Now the deeds of the flesh are evident, which are: immorality, impurity, sensuality,
idolatry, sorcery, enmities, strife, jealousy, outbursts of anger, disputes, dissensions, factions,

Rev 18:23 — and the light of a lamp will not shine in you any longer; and the voice of the bridegroom and bride will not be heard in you any longer; for your merchants were the great men of the earth, because all the nations were deceived by your sorcery.

Rev 9:21 — and they did not repent of their murders nor of their sorceries nor of their immorality nor of their thefts.

Rev 21:8 — ~"But for the cowardly and unbelieving and abominable and murderers and immoral persons and sorcerers and idolaters and all liars, their part {will be} in the lake that burns with fire and brimstone, which is the second death."

Rev 22:15 — Outside are the dogs and the sorcerers and the immoral persons and the murderers and the idolaters, and everyone who loves and practices lying.

Gen. 40:8 — Then they said to him, "We have had a dream and there is no one to interpret it." Then Joseph said to them, "Do not interpretations belong to God? Tell {it} to me, please."

Ex 22:18 — "You shall not allow a sorceress to live.

Leviticus 19:31 — `Do not turn to mediums or spiritists; do not seek them out to be defiled by them. I am the LORD your God.

2 Kings 23:24 — Moreover, Josiah removed the mediums and the spiritists and the teraphim and the idols and all the abominations that were seen in the land of Judah and in Jerusalem, that he might confirm the words of the law which were written in the book that Hilkiah the priest found in the house of the LORD.

1 Chron. 10:13-14 — So Saul died for his trespass which he committed against the LORD, because of the word of the LORD which he did not keep; and also because he asked counsel of a medium, making inquiry {of it,}
and did not inquire of the LORD. Therefore He killed him and turned the kingdom to David the son of Jesse.

Isaiah 8:19 — When they say to you, "Consult the mediums and the spiritists who whisper and mutter," should not a people consult their God? {Should they} {consult} the dead on behalf of the living?

Isaiah 19:3 — "Then the spirit of the Egyptians will be demoralized within them; And I will confound their strategy, So that they will resort to idols and ghosts of the dead And to mediums and spiritists.