Type string introduces depedency on Python >=3.9

[EricGallimore]

With #886, a change was made to ros_loader.py that uses the dict[str, Any] and tuple[str, str] type hint syntax. This, unfortunately, requires a recent version of Python.

Jazzy still requires Python 3.8.3 on Windows; I'm not sure if it is a problem on any other platforms.

https://github.com/RobotWebTools/rosbridge_suite/blame/d22f102b59e7d9fdeea0ec5e74aa8b98358585d7/rosbridge_library/src/rosbridge_library/internal/ros_loader.py#L132

This could be fixed in ros_loader.py by using

from typing import Dict, Tuple

and then

Dict[str, Any]
Tuple[str, str]

in place of dict[str, Any] and tuple[str, str]