core: migrate code to benefit from 3.9 stuff

for now keeping ruff on 3.8 target version, need to sort out modules as well

my/core/pandas.py

@@ -14,9 +14,7 @@ from typing import (
     TYPE_CHECKING,
     Any,
     Callable,
-    Dict,
     Literal,
-    Type,
     TypeVar,
 )
 
@@ -177,7 +175,7 @@ def _to_jsons(it: Iterable[Res[Any]]) -> Iterable[Json]:
 Schema = Any
 
 
-def _as_columns(s: Schema) -> Dict[str, Type]:
+def _as_columns(s: Schema) -> dict[str, type]:
     # todo would be nice to extract properties; add tests for this as well
     if dataclasses.is_dataclass(s):
         return {f.name: f.type for f in dataclasses.fields(s)}  # type: ignore[misc]  # ugh, why mypy thinks f.type can return str??