ORM: Sealable.seal() return self instead of None

The `Sealable` mixin is used by the `ProcessNode` which allows it to be
sealed. By having the `seal` method return `self`, which will be the
`ProcessNode` instance, it brings the behavior on par with the `store`
method.

aiida/orm/utils/mixins.py

@@ -192,11 +192,13 @@ def is_sealed(self) -> bool:
         """Returns whether the node is sealed, i.e. whether the sealed attribute has been set to True."""
         return self.base.attributes.get(self.SEALED_KEY, False)  # type: ignore[attr-defined]
 
-    def seal(self) -> None:
+    def seal(self) -> 'Sealable':
         """Seal the node by setting the sealed attribute to True."""
         if not self.is_sealed:
             self.base.attributes.set(self.SEALED_KEY, True)  # type: ignore[attr-defined]
 
+        return self
+
     @override
     def _check_mutability_attributes(self, keys: list[str] | None = None) -> None:  # pylint: disable=unused-argument
         """Check if the entity is mutable and raise an exception if not.