Use context managers in test functions

Elimiinate 32 of 70 of the following warnings emitted while running unit
tests: `RuntimeWarning: deallocating CachingFileManager`

The remaining 38 appear to all be related to the HDF5VirtualBackend (see
#468)

Fixes #390

Author: chuckwondo

conftest.py

@@ -61,7 +61,7 @@ def netcdf4_file_with_data_in_multiple_groups(tmp_path: Path) -> str:
 
 
 @pytest.fixture
-def netcdf4_files_factory(tmp_path: Path) -> Callable:
+def netcdf4_files_factory(tmp_path: Path) -> Callable[[], tuple[str, str]]:
     def create_netcdf4_files(
         encoding: Optional[Mapping[str, Mapping[str, Any]]] = None,
     ) -> tuple[str, str]:
@@ -96,7 +96,8 @@ def netcdf4_file_with_2d_coords(tmp_path: Path) -> str:
 def netcdf4_virtual_dataset(netcdf4_file):
     from virtualizarr import open_virtual_dataset
 
-    return open_virtual_dataset(netcdf4_file, indexes={})
+    with open_virtual_dataset(netcdf4_file, indexes={}) as ds:
+        yield ds
 
 
 @pytest.fixture

virtualizarr/backend.py

@@ -109,9 +109,9 @@ def open_virtual_dataset(
     cftime_variables: Iterable[str] | None = None,
     indexes: Mapping[str, Index] | None = None,
     virtual_array_class=ManifestArray,
-    virtual_backend_kwargs: Optional[dict] = None,
-    reader_options: Optional[dict] = None,
-    backend: Optional[VirtualBackend] = None,
+    virtual_backend_kwargs: dict | None = None,
+    reader_options: dict | None = None,
+    backend: type[VirtualBackend] | None = None,
 ) -> Dataset:
     """
     Open a file or store as an xarray Dataset wrapping virtualized zarr arrays.