Change HelpTheme type to allow sub-classing

[kdeldycke]

In Click Extra, I have a default theme lying around that try to extends cloup.HelpTheme with additional categories of styles. Like log-level colors and other help screen details.

The problem is that cloup.HelpTheme is a NamedTuple. And I cannot easily sub-class it. As you can see in my code, I had to redefine the whole properties and methods.

I also encountered issues with MyPy which cannot solve references to sub-styles:

click_extra/version.py:58: error: Cannot determine type of "default_theme"  [has-type]
        default_package_name_style: IStyle | None = default_theme.invoked_command
                                                    ^~~~~~~~~~~~~
click_extra/version.py:59: error: Cannot determine type of "default_theme"  [has-type]
        default_prog_name_style: IStyle | None = default_theme.invoked_command
                                                 ^~~~~~~~~~~~~

That is somewhat expected, as NamedTuple subclasses is flagged as wontfix by Python's typing project.

@janluke, would you be open to change the type of HelpTheme so I can reuse it seamlessly?

I naively think that a class is the natural choice, but maybe a @dataclass might be another alternative to NamedTuple. I'm pretty sure you have a better idea of what HelpTheme should look like in that case.

[janluke]

Weird I used a NamedTuple for HelpTheme. Whatever was my reasoning, it was a bad idea. I'll have a look when I have time.

[janluke]

I'll fix this next later today. I'll probably have to bump the major version for this.

[kdeldycke]

Thanks @janluke for this refactor! 💪