Hi, 

Try monkey patch:

from odoo import fields.Many2many as Many2many_class

def function_name_as_you_like_it_to_be(..):
  // Your code here

Many2many_class.odoo_function_you_want_to_override() = function_name_as_you_like_it_to_be

----------------------------------

your  code should look like this, using methode "groupable" as an example:

from odoo import fields.Many2many as Many2many_class

@property
def groupable(self):
    //my code here   
    return self.store 

Many2many_class.groupable = groupable

hope this helps.

Upvote if it does or write back for further analysis.

Regards.