Allow template_folder argument to be os.PathLike[str] as is allowed in Flask

[502E532E]

Quart currently supports only str | None for the template folder argument of the app, while Flask also supports os.PathLike[str].

quart/src/quart/app.py

Line 287 in b5593ca

template_folder: str | None = "templates",

flask/src/flask/app.py

template_folder: str | os.PathLike[str] | None = "templates",

As Quart seems to pass the argument directly to Flask.App, I do not think the further restriction is necessary.