literals_interactions.py: do not mandate optional narrowing behaviour from type checkers

[AlexWaygood]

X-ref #2208 (comment).

An example in this file mandates that type checkers should narrow an object x of type str to Literal["foo"] after an if x == "foo" check. This example appears to be testing adherence to the passage in the spec at https://typing.python.org/en/latest/spec/literal.html#interactions-with-narrowing. But the passage in the spec here clearly indicates that narrowing str or LiteralString types to Literal strings is optional:

Type checkers may optionally perform additional analysis for both enum and non-enum Literal types beyond what is described in the section above.

For example, it may be useful to perform narrowing based on things like containment or equality checks

The narrowing examples given by the spec are also unsound; it's easy to create a str subclass (and str subclasses are common, due to StrEnum in the standard library) that has custom equality semantics and breaks the assumptions of narrowing like this:

>>> class Foo(str):
...     def __eq__(self, other):
...         return other == "Foo"
... 
>>> Foo("Bar") == "Foo"
True

This PR keeps a version of the existing test (since it comes directly from the spec), but allows type checkers to optionally emit errors on certain lines if they do not implement the optional narrowing behaviour described by the spec. It also adds additional tests using LiteralString, for which narrowing to a Literal string type via equality or containment is unambiguously sound.

The effect of this PR is that mypy and ty now fully pass literals_interactions.py. All other type checkers have their conformance score unchanged.

[JelleZijlstra]

For usability reasons I think it is more practical to narrow in these cases, but you are technically correct it is unsound.