How authentic is the Hadith: "Whoever learns something of sorcery, be it little or much, his last covenant with Allah is severed, so his punishment is execution unless he repents," and the other narration without "unless he repents"? Is there repentance for learning sorcery and for its practitioner?

The chain of narration is weak, and the report regarding magic narrated by Abd al-Razzaq is mursal (missing a link in the chain of narration back to the Prophet) and its isnad (chain of narration) is very weak, so it cannot be used as proof. As for the repentance of a magician:

1. Between him and Allah Almighty: The repentance of a magician is accepted, like that of other sinners, even if his sin was shirk (polytheism), as evidenced by the acceptance of the repentance of Pharaoh's magicians.

2. Before a judge or ruler: This issue is a matter of scholarly disagreement. Some scholars are of the opinion that he should not be asked to repent, due to the uncertainty of the sincerity of his repentance. Others are of the opinion that he should be asked to repent and his repentance accepted, because it is no greater than shirk, and Allah accepts repentance.