Closes#5644. Part of #3317.
22 `aqt` modules already type-check cleanly under `strict_optional`, so
this opts them in. Config only — no code changes.
Verified clean with `mypy --platform` for `darwin`, `linux` and `win32`.
`aqt.mpv` is left out: it sets `ignore_errors = True`, so a stanza there
would be a no-op.
