Inventory items should not allow selecting itself as parent

[drmsoffall]

NetBox version

v3.2.0

Python version

3.8

Steps to Reproduce

1. Create a new device
2. Create an inventory item
3. Edit inventory item
4. Select same inventory item in Parent dropdown
5. Click Save

Expected Behavior

Inventory item does not list itself in 'Parent' dropdown or, if not feasible, toast error instead of exception.

Observed Behavior

Server error:

<class 'mptt.exceptions.InvalidMove'>

A node may not be made a child of itself.