cext: add .pyi files for C extensions I'm unsure if all the annotations are completely accurate. But having these helps type checkers reason about our C extensions. Differential Revision: https://phab.mercurial-scm.org/D9281

from typing import Optional

version: int

def b85encode(text: bytes, pad: Optional[int]) -> bytes: ...
def b85decode(text: bytes) -> bytes: ...