Protect zarr import

virtualizarr/writers/icechunk.py

@@ -4,7 +4,6 @@
 from xarray import Dataset
 from xarray.backends.zarr import encode_zarr_attr_value
 from xarray.core.variable import Variable
-from zarr import Group  # type: ignore[import-untyped]
 
 from virtualizarr.manifests import ChunkManifest, ManifestArray
 from virtualizarr.zarr import encode_dtype
@@ -36,6 +35,7 @@ def dataset_to_icechunk(ds: Dataset, store: "IcechunkStore") -> None:
     store: IcechunkStore
     """
     from icechunk import IcechunkStore  # type: ignore[import-not-found]
+    from zarr import Group  # type: ignore[import-untyped]
 
     if not isinstance(store, IcechunkStore):
         raise TypeError(f"expected type IcechunkStore, but got type {type(store)}")