Fix type annotations for converter types, considering the Converter class

[filbranden]

Summary

A converter can either be a callable with a single argument or an instance of the Converter class, fix the type annotation to allow both cases everywhere _ConverterType is used

Also update field() and attr.ib() which are able to take a list or tuple of converters as an implicit pipe, add type annotations for that syntax

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[filbranden]

@hynek Would you kindly take a look whenever possible? Thanks!

[Tinche]

LGTM from what I can follow

[hynek]

It looks to me like worthy a changelog entry because apparently converters are better supported now? 🤔

[filbranden]

I believe Converters are already mentioned in the changelog for 24.1.0:

"It is now possible to wrap a converter into an attrs.Converter and get the current instance and/or the current field definition passed into the converter callable.

Note that this is not supported by any type checker, yet. #1267"

This just fixes some type annotations but I believe full type checker support would need some mypy changes (see commented out code) so not sure how much is worth mentioning in a changelog.

Having said that, feel free to add a note if you'd like 😃