types: fix variance issues and improve type coverage

[jorenham]

The generic types in click.types were all parametrized using an invariant type parameter. This can easily lead to unexpected type errors for downstream users. For example, type-checkers would not allow you to assign ParamType[bool] to an ParamType[int], even though bool is a subclass of int. This addresses these issues by using appropriate variance for these generic type parameters.

This also fixes several other typing issues, removing several # type: ignores and t,casts.

I also took the parametrize some of the typed dicts, avoid several (intrinsically type-unsafe) Any annotations.

And just like #3422, this also fills in the missing annotations of attributes and constants. This consequently increases the type coverage by 2.18% (91.5% -> 93.68%).

I realize there are quite a lot of changes here. But I was afraid that spreading these out over multiple PRs would result in a merge conflict mess. But if you want me to split this up anyway, then that's also fine as far as I'm concerned.

[Rowlando13]

I don't have enough knowledge of typing to review these changes. @kdeldycke

[kdeldycke]

I don't have enough knowledge of typing to review these changes. @kdeldycke

Same thing here, my brain breaks a bit with these kind of high-level typing. So no strong technical opinion on my side: the tests pass, the type checker is happy, the code does not introduce any malware, is purely type checking, and @jorenham already demonstrated reactivity and seriousness. So based on these circumstantial evidences, I would merge it and go ahead! Let's clear the backlog! 🤠