incorrect Argentinian CUIT .format()

[eamanu]

Hi,

This issue was reported by @eduzen on [1].

I am not a expert but reading [2] and this not official webpage [3], the CUIT is composed for [2 digits]-[8 digits]-[1 digit]: ##-########-#.

And a CUIT just accept numbers (and 2 hyphen).

Also, the first 2 digits just accept some numbers listed on [3] (20, 27, 23, 24, 25, 26) and some others for international purpose (50, 51, 55).

I can work on a patch for this issue if there are not oppositions :)

[1] https://github.com/PyAr/asoc_members/pull/150#discussion_r361808212 [2] https://es.wikipedia.org/wiki/Clave_%C3%9Anica_de_Identificaci%C3%B3n_Tributaria [3] https://www.serautonomo.net/argentina/que-es-el-cuit.html