weboob.browser.browsers: no pickling when saving cookie jar in StatesMixin
pickle may be unsafe for loading data. All we want is a cookie jar, we can just serialize its cookies, not necessarily the whole jar with its type and policy. Old format is base64(compress(pickle(jar))). New format is base64(compress(json(jar))) where JSON conversion is on the list of cookies (taking name, value, domain, path, secure (https) and expires). dump_state will now save using the new format while load_state supports both new and old format. This allows compatibility for some time, then later old format support in load_state will be dropped too.
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