…s.KW_ONLY.

* Respects KW_ONLY for dataclass attributes that don't use dataclasses.field.
* Removes unnecessary version check for applying kwonly-ness: it's impossible
  for a user to even set KW_ONLY or kw_only unless they're using a version
  where the field/parameter is available, so once it's been set, we can safely
  assume we're in a high-enough version.

PiperOrigin-RevId: 597970055

* Reports an [invalid-annotation] error when a TypedDict key has an ambiguous
  type.
* Simplifies some of the TypedDict code by assuming that keys have unambiguous
  types.

Fixes #1566.

PiperOrigin-RevId: 599320573