D7380: encoding: define per-use identity functions only in typechecking mode

[Yuya Nishihara]

>     strtolocal = pycompat.identity  # type: (str) -> bytes
>     strfromlocal = pycompat.identity  # type: (bytes) -> str

`# type: Callable[[str], bytes]` might work. pytype complained
`# type: ... -> ...` is an invalid type comment.