mamalon.py

from collections import defaultdict                                                                                                                           
magia = defaultdict(self.env['account.tax'])
for i in self.tax_line_ids:
    magia[i.tax_group_id.name] += i

@josemoralesp what about use also dict.fromkeys()?

magia = dict.fromkeys((self.env['account.tax'],))
for i in self.tax_line_ids:
    magia[i.tax_group_id.name] += i

UPDATE

Better to use defaultdict instead of dict.fromkeys(): the first one if a key does not exists wont be turn a KeyError that the second one does