mail: add type hints for pytype We essentially annotate functions in which handling of bytes/str is not obvious in order to hopefully clear things out. See also changeset 2ade00f3b03b introducing typing hints in Mercurial. Most types are straightforward but a few is wrong, and we need to either disable pytype on respective instructions or use wrong annotations. These will be fixed in next changesets. Notice the type Union[bytes, str] of "s" parameter of headencode(), this reflects how email.header.Header.append() behaves.