This is a regression from 7ca89f56ee, which introduced StoredList.
The newly added test was failing without the change.
```
Traceback (most recent call last):
File "/home/user/wspace/electrum/electrum/gui/qt/main_window.py", line 1786, in new_contact_dialog
self.set_contact(line2.text(), line1.text())
File "/home/user/wspace/electrum/electrum/gui/qt/main_window.py", line 1435, in set_contact
self.contacts[address] = ('address', label)
File "/home/user/wspace/electrum/electrum/contacts.py", line 75, in __setitem__
self.save()
File "/home/user/wspace/electrum/electrum/contacts.py", line 62, in save
self.db.put('contacts', dict(self))
File "/home/user/wspace/electrum/electrum/json_db.py", line 42, in wrapper
return func(self, *args, **kwargs)
File "/home/user/wspace/electrum/electrum/json_db.py", line 318, in put
self.data[key] = copy.deepcopy(value)
File "/usr/lib/python3.10/copy.py", line 146, in deepcopy
y = copier(x, memo)
File "/usr/lib/python3.10/copy.py", line 231, in _deepcopy_dict
y[deepcopy(key, memo)] = deepcopy(value, memo)
File "/usr/lib/python3.10/copy.py", line 172, in deepcopy
y = _reconstruct(x, memo, *rv)
File "/usr/lib/python3.10/copy.py", line 271, in _reconstruct
state = deepcopy(state, memo)
File "/usr/lib/python3.10/copy.py", line 146, in deepcopy
y = copier(x, memo)
File "/usr/lib/python3.10/copy.py", line 231, in _deepcopy_dict
y[deepcopy(key, memo)] = deepcopy(value, memo)
File "/usr/lib/python3.10/copy.py", line 172, in deepcopy
y = _reconstruct(x, memo, *rv)
File "/usr/lib/python3.10/copy.py", line 271, in _reconstruct
state = deepcopy(state, memo)
File "/usr/lib/python3.10/copy.py", line 146, in deepcopy
y = copier(x, memo)
File "/usr/lib/python3.10/copy.py", line 231, in _deepcopy_dict
y[deepcopy(key, memo)] = deepcopy(value, memo)
File "/usr/lib/python3.10/copy.py", line 161, in deepcopy
rv = reductor(4)
TypeError: cannot pickle '_thread.RLock' object
```
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