In this diff:
* Loosen `set.__[i]sub__()` to allow typical use cases (that work at runtime). Namely, allow removing `unicode` from a set of `str`, and allow removing optional values from non-optional sets.
* Avoid using union return types in `cryptography` deserialization functions.
* Tune `SupportsItems` so that `dict` implements it on Python 2.
Co-authored-by: Ivan Levkivskyi <ilevkivskyi@dropbox.com>