[web/session] make sure not to modify an inner value of session.data (closes #11217958) When we modify a mutable value stored in the session's data (dict-like object) we prevent an alternative session data handler (eg. pyramid-redis-session) from detecting this update and synchronizing it in its backend (like redis).

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if repo.system_source.dbdriver == 'postgres':

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    sql('ALTER TABLE appears ADD COLUMN weight float')

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    sql('UPDATE appears SET weight=1.0 ')