create _arr accessor method automagically

[martinberoiz]

Suggestion:

I propose that instead of the user having to add the accessor attribute _arr to the class, the property is automatically added to the class scope on creation.

[leliel12]

uttrs is just a simple extension of attrs. Essentially it provides two functions that generate preconfigured attributes. But at no time will it interfere with the original operation of @ attr.s. So I don't have access to the class where these attributes are added. To achieve this objective it would be necessary to implement #5