allow arbitrary types in set difference

[randolf-scholz]

Fixes #9004
See also: #11403

This PR allows set differences with disjoint generic types. This option was previously argued against (#9004 (comment)) because

A user shipped code that did set[bytes] - set[str] to production, and of course, it didn't work as expected (#1840). There is no use case for set[int] - set[str]: if your code does it, that's very likely not intentional.

However, typeshed shouldn't try to act as a linter. That's the job of the type checker. There are cases when the current setup hurts, for instance when we try to do set[Literal["a", "b"]] - set[str].

This should be handled on the type-checker side, for instance by warning on set[T] - set[S] if T and S have empty intersection (⇝ disjoint_bases https://peps.python.org/pep-0800/), which would catch the bad case described above.

[github-actions]

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

pandas (https://github.com/pandas-dev/pandas)
+ pandas/core/dtypes/cast.py:870: error: Unused "type: ignore" comment  [unused-ignore]
+ pandas/core/dtypes/cast.py:871: error: Unused "type: ignore" comment  [unused-ignore]

ibis (https://github.com/ibis-project/ibis)
- ibis/backends/sql/datatypes.py:1398: error: Unsupported operand types for - ("set[type[Never]]" and "set[type[SqlglotType]]")  [operator]
- ibis/expr/datatypes/tests/test_core.py:581: error: Unsupported operand types for - ("set[type[Never]]" and "set[AnnotableMeta]")  [operator]

schemathesis (https://github.com/schemathesis/schemathesis)
- src/schemathesis/config/_error.py:54: error: Argument 1 to "set" has incompatible type "Any | Unset"; expected "Iterable[Any | None]"  [arg-type]
+ src/schemathesis/config/_error.py:54: error: Argument 1 to "set" has incompatible type "Any | Unset"; expected "Iterable[object]"  [arg-type]

pydantic (https://github.com/pydantic/pydantic)
- pydantic/v1/main.py:907: error: Set comprehension has incompatible type Set[int | str]; expected Set[str | None]  [misc]
- pydantic/v1/main.py:907: note: Left operand is of type "set[str] | dict_keys[str, Any]"