Change type of layer Metadata from Dict to Mapping (#357)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Draga Doncila Pop <[email protected]>

src/npe2/types.py

@@ -1,7 +1,7 @@
+from collections.abc import Mapping
 from typing import (
     TYPE_CHECKING,
     Callable,
-    Dict,
     List,
     Literal,
     NewType,
@@ -48,7 +48,7 @@ def __array__(self) -> "np.ndarray":
 LayerName = Literal[
     "graph", "image", "labels", "points", "shapes", "surface", "tracks", "vectors"
 ]
-Metadata = Dict
+Metadata = Mapping
 DataType = Union[ArrayLike, Sequence[ArrayLike]]
 FullLayerData = Tuple[DataType, Metadata, LayerName]
 LayerData = Union[Tuple[DataType], Tuple[DataType, Metadata], FullLayerData]