It’s one of the Big Apple’s best kept secrets. A tiny neighborhood in the North Bronx offers a handful of lucky New Yorkers an upscale and tranquil suburban lifestyle — with cut-rate city property tax bills. About three dozen homes on a strip of land north of Pelham Bay Park get to send their kids to high-achieving Westchester County schools and live in suburban bliss, all while paying a fraction of the taxes their neighbors across the street pay. “It’s an anomaly — no one is quite sure why it