fix: release with python3.8

Context

There are issues in the aidge_core CI Release with Python3.8 like "TypeError: 'type' object is not subscriptable"

• Python3.8 doesn't support the newer type hinting syntax, requiring from typing import Optional, Union, List instead
• The | syntax for union types was introduced in Python3.10
• is_relative_to() method was introduced in Python 3.9

This MR aims at fixing these.