typing: add type annotations to mercurial/i18n.py I'm a little unsure of this because `gettext()` clearly allows for passing unicode. But the comments seem to indicate that this is related to tests, and this was useful for catching unicode being passed to `_()` in the keyring extension. I'm also not sure why `_(None)` would make any sense, so maybe the argument shouldn't be optional? I didn't add it to the lambda in plain mode because that spilled beyond 80 characters and so black mangled it. Black and pytype disagree on where the comment to disable a check needs to go, so this has to disable and then enable the checking. Differential Revision: https://phab.mercurial-scm.org/D10124