Fixes #3430 (serializing an easyguide corrupts it).

[BenZickel]

• The issue is that on Python 3.12 the super() object of easyguide.Group started having the __getstate__ attribute (this doesn't happen in python 3.9), which is then called to return the state object, which is then modified without making a copy before modification, which causes the corruption.
• The solution is to ensure that we always make a copy of the state object before modifying it.
• This was tested on both python 3.9 and 3.12.

[fritzo]

Nice work figuring this out!