Support for attrs

[9ao9ai9ar]

attrs is the inspiration for and the continuously improved version of the Python dataclasses with many additional features and almost 4 hundred million downloads last month, according to ClickPy. I would like to request that two new output model types be added:

1. attrs.define for generating attrs classes using its new API, and
2. attr.dataclass as a drop-in replacement of dataclasses.dataclass for better interop with attrs.

The latter output type should be easy to implement, as currently I only need to monkey patch one line in the generated file: change from dataclasses import dataclass to from attr import dataclass. But this tiny change allows, for example, attrs.fields(C) to list the attrs attributes of the decorated class C, and once #2437 lands, more customization options would become available for free.

[9ao9ai9ar]

Case 2 above can already be achieved by setting additional-imports = "attr.dataclass", which adds the line from attr import dataclass after from dataclasses import dataclass to override it.