Typehints for fetchItem(s) in base.py

[Dr-Blank]

Description

make mypy pass for these methods

related #1296

Checklist:

• [ ] #1302 is complete
• [ ] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas