“Saint Omer,” one of 2022′s near-perfect films, marks the narrative feature debut of the French documentary filmmaker Alice Diop. Set mostly in the northeastern French town of the title, it concerns a Paris professor following a murder trial there as primary research for her next book. That book, whose working title (which her publisher doesn’t like) is “Medea Castaway,” ties into the legacy of the Euripidean Medea myth and a forbidding question for the ages: How does a dramatist effectively humanize the act of infanticide? “A woman who has killed her baby can’t really expect any sympathy,” sa…