Incorrect typing using AnyStr

[Brandieee]

AnyStr in the current code base is used incorrectly, almost as directly outlined in the documentation how not to use it.
https://docs.python.org/3.14/library/typing.html#typing.AnyStr

It is also deprecated as of python 3.13

It should be replaced with the correct str | bytes union instead.

Environment:

• Python version: >3.9
• Quart version: 0.20.0

[Brandieee]

Issue is mentioned in #375 as well