Idea: detailed version-specific type annotations for `Schema`

[Zac-HD]

I recently saw python/typing#182 (comment) proposing a nice trick for type-checking JSON data, and it occured to me that this could be extended to define version-specific Schema types using TypedDict. See also python-jsonschema/jsonschema#766 and python-jsonschema/jsonschema#778 - if we get a working implementation, this would probably be suitable for upstreaming.

For example, this would allow for static detection of exclusiveMaximum as either bool or int | float depending on the schema draft version.

[Zac-HD]

Mypy 0.990 has native support for recursive types, so this is now possible without hacks.