Loosen some types to avoid pain in real-life situations

[ilevkivskyi]

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.

About the first item, I think mypy should support the overloads on generic self-types (see https://mypy.readthedocs.io/en/stable/more_types.html#restricted-methods-in-generic-classes), but I am not sure about other type-checkers.

cc @JukkaL

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

core (https://github.com/home-assistant/core.git)
- homeassistant/helpers/device_registry.py:597: error: Unsupported operand types for - ("Set[str]" and "Set[Optional[str]]")  [operator]

[ilevkivskyi]

Hm, mypy thinks the overloads are unsafe, I will now try to tweak or ignore to make the build clean.

[srittau]

LGTM, apart from the build failure.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

core (https://github.com/home-assistant/core.git)
- homeassistant/helpers/device_registry.py:597: error: Unsupported operand types for - ("Set[str]" and "Set[Optional[str]]")  [operator]

[hauntsaninja]

For posterity / to keep things linked, set difference was strict-ified in #3886