Declare Variable to be unhashable

The appropriate place to do this seems to be beside the definition
of `__eq__` since defining `__eq__` without `__hash__` renders an
object unhashable.

For a reference to how types are marked as non-hashable, see
<https://github.com/python/typeshed/pull/3219/files>

xarray/core/_typed_ops.py

@@ -632,6 +632,8 @@ def __eq__(self, other: VarCompatible) -> Self:
     def __eq__(self, other: VarCompatible) -> Self | T_DataArray:
         return self._binary_op(other, nputils.array_eq)
 
+    __hash__: None  # type:ignore[assignment]
+
     @overload  # type:ignore[override]
     def __ne__(self, other: T_DataArray) -> T_DataArray:
         ...