Fix 2 type annotations in dataarray.py (#5548)

Both `set_index` and `reset_index` are wrappers to other methods that return `"DataArray"`, not `Optional["DataArray"]`. That is, they will never return None. That's why these methods should also have only `"DataArray"` in there return signature. This way it will be possible to do something like `myarray = myarray.reset_index(...)` without getting a complaint from Mypy.

For extended discussion, see #5533 (comment)

xarray/core/dataarray.py

@@ -1902,7 +1902,7 @@ def set_index(
         indexes: Mapping[Hashable, Union[Hashable, Sequence[Hashable]]] = None,
         append: bool = False,
         **indexes_kwargs: Union[Hashable, Sequence[Hashable]],
-    ) -> Optional["DataArray"]:
+    ) -> "DataArray":
         """Set DataArray (multi-)indexes using one or more existing
         coordinates.
 
@@ -1958,7 +1958,7 @@ def reset_index(
         self,
         dims_or_levels: Union[Hashable, Sequence[Hashable]],
         drop: bool = False,
-    ) -> Optional["DataArray"]:
+    ) -> "DataArray":
         """Reset the specified index(es) or multi-index level(s).
 
         Parameters