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

Pull Request Check List

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• [x] Added tests for changed code. Our CI fails if coverage is not 100%.
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• [x] Changes or additions to public APIs are reflected in our type stubs (files ending in .pyi).
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[filbranden]

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