Many of the methods, especially the builder methods (limit, clone, _query_spec, sort, where, ..., of AsyncCursor/Cursor differ only in their return or argument type annotations (async or sync versions of classes). By using typing.Self as the return type and _Agnostic* in parameter types, we can consolidate these methods into the shared _AgnosticCursor base class.

Note that typing.Self was added in Python 3.11, so older versions of Python will have to import from typing_extensions under a if TYPE_CHECKING guard.