gh-104683: Argument clinic: modernise cpp.Monitor

[AlexWaygood]

• Convert the class into a dataclass (this necessitates moving is_a_simple_defined into the global namespace)
• use an f-string for the __repr__ method

• Issue: Modernise code in Tools/clinic/ #104683

[AlexWaygood]

I think this is the last class that can be usefully converted to become a dataclass, FTR. For the remaining classes in argument clinic, ISTM converting the classes to become dataclasses would make the code more complicated, rather than improving readability. And that would defeat the object.

[erlend-aasland]

Great, thanks!

[erlend-aasland]

• Convert the class into a dataclass (this necessitates moving is_a_simple_defined into the global namespace)

Makes more sense to have it there as a global, IMO