Here's what I did for those who want to do the same thing:

class MailActivity(models.Model):

    _inherit = ['mail.thread','mail.activity']
    _name='mail.activity'

    @api.model
    def create(self, values):
        rec = super(MailActivity, self).create(values)
        model= self.env[rec.res_model].search([('id','=',rec.res_id)])
        followers=model.message_partner_ids
        follower_ids=(tuple(followers.ids))
        rec.message_post(subject='New activity',type='email',partner_ids=follower_ids)
        return rec