Split stdlib into Python 2 and 3 versions (#5442)

All new files in stdlib/@python2 are straight copies of the
corresponding files in stdlib.

stdlib/@python2/antigravity.pyi

@@ -0,0 +1,4 @@
+import sys
+
+if sys.version_info >= (3, 0):
+    def geohash(latitude: float, longitude: float, datedow: bytes) -> None: ...