I saw "Pythonic Sorcery" mentioned on a list of things that would merit the death penalty in ancient Jerusalem. But, when I went to Google it, all I found are...more readings about the capital punishment in Judaism. So what is it? Where does this concept originate?
"Pythonic sorcery" appears to be how Wikipedia translates "Yid'oni" (ידעוני) (comp. the Hebrew in Rambam's list), although it's possible they were also referring to the Ba'al Ov, another form of necromancy. According to the essay brought by @ezra, pythonic sorcery is comprised of different forms of sorcery and divination done with relation to snakes. The Witch of Endor was called by classical and medieval Christians a "Pythonissa" which, according to this entry, related her to the priestess of Apollo (better known as "the Oracle of Delphi"):
"The priestess of Apollo was called Pythia. The witch of Endor is called mulier Pythonem habens, "a woman having the python," I Kings 28:7, indicating that she had some aspects of the pagan priestess. Isidore, in his discussion of magicians, says that the woman who called up Samuel was Pythonissa (Etym VIII.ix.7, 21).
The fiend explains to the summoner that sometimes devils arise with dead bodies and speak as reasonably and as fairly as Samuel did to the Phitonissa, FrT 1506-1511..."
This interpretation, which connects different forms of necromancy with snakes, isn't too far-off from the Talmud's explanation of the Yid'oni, which involved calling forth spirits through a bone of a mysterious animal called a "Yaddua" (see here and here).
My own guess is that this Greco-Roman term is the origin of the word "Pitom" which is used by our sages to describe such necromancers (see for example Mishnah Sanhedrin 7:7). Compare especially the Latin "Pythonem", which has an "M" at the end. And it seems that the Aruch may have also thought so, or something similar.
It is known that the early Church Fathers (Isidore being the last of them) learned many Jewish traditions - not all of which survived in Jewish texts we have today - from local Jews, including Tannaim and Amoraim (see Jerome here for example). It would not be far-fetched to say that Isidore's tradition of the Ba'alat Ov from the time of Shaul being some sort of pitom-serpent sorceress came from Jewish tradition. As for the connection to Apollo, let's not forget that the Plishtim, who most likely originated in the Grecian Mycenaean culture, neighbored Yisrael for many centuries. It is possible that Israelite sorcerers received magical knowledge from them, thus resulting in the connection.