typing: add type hints to most mercurial/pycompat.py functions The `rapply` methods are left out because it's not `rapply(f, xs: _T0) -> _T0` as I first thought- it's used somewhere to walk a collection and convert between bytes and str. Also, the `open()` call is partially untyped because I'm not sure what its purpose is at this point- both the name and mode can be either bytes or str as it is currently constituted. It might make sense to assert that the file is being opened in binary mode (like `namedtempfile()`) and cast the result to `BinaryIO`, but that shouldn't be smuggled in with these other changes. The return is currently typed as `Any` because something suddenly got smarter and a few uses in util.py (like readfile()) suddenly think it returns `IO[str]` instead of `IO[bytes]` (BinaryIO), and it flags the type mismatch there.