Use typing Union instead of | to support Python<3.10

[Hansimov]

Issue Description

The union operator | suggested in PEP 604 is introduced to Python >= 3.10.
So this syntax is not compatible with Python < 3.10.

Solution

Use Union[X, Y] to replace X | Y.
See more details in Python doc for typing.Union.

Changes Example

In src/EdgeGPT/conversation.py:

+ from typing import Union

@@ -10,9 +10,9 @@
class Conversation:
    def __init__(
        self,
--        proxy: str | None = None,
++        proxy: Union[str, None] = None,