metadata on a schema can be passed as str (#128)

For details see https://github.com/apache/arrow/blob/apache-arrow-17.0.0/python/pyarrow/types.pxi\#L2053-L2056

pyarrow-stubs/__lib_pxi/types.pyi

@@ -586,7 +586,7 @@ def ensure_type(ty: Literal["duration[ns]"]) -> DurationType[Literal["ns"]]: ...
 def ensure_type(ty: Literal["month_day_nano_interval"]) -> MonthDayNanoIntervalType: ...
 def schema(
     fields: Iterable[Field] | Iterable[tuple[str, DataType]] | Mapping[str, DataType],
-    metadata: dict[bytes, bytes] | None = None,
+    metadata: dict[bytes | str, bytes | str] | None = None,
 ) -> Schema: ...
 def from_numpy_dtype(dtype: np.dtype) -> DataType: ...
 def is_boolean_value(obj: Any) -> bool: ...