Steps to reproduce

1. Have a bot using PicklePersistence with singlefile=True

2. Upgrade to v12.4

3. restart bot

Expected behaviour

pickled file is read correctly

Actual behaviour

key error bot_data is thrown

Current workaround:

Add an empty dict bot_data to the file manually. Quick and dirty script:

import pickle

filename = 'my_pickle_persistence_file'

with (open(filename, 'rb')) as file:
    data = pickle.load(file)

data['bot_data'] = {}

with open(filename, 'wb') as f:
    pickle.dump(data, f)

Will be closed by #1760