Don’t let the title fool you — Rivka Galchen’s second novel “Everyone Knows Your Mother Is a Witch” is as far from contemporary popular fantasy as the Earth is from the sun. Based on an assemblage of historical documents from the seventeenth century, “Everyone Knows Your Mother Is a Witch” is a fictionalized account of the real-life story of astronomer Johannes Kepler’s mother, Katharina Guldenmann Kepler, an accused — and acquitted — witch.