[schema] restore constraint checking when running on old sqlite Old sqlite3 doesn't provide CHECK constraint names in error messages, preventing us from translating a backend integrity error into a ValidationError. This was added in 2012, but the sqlite3 version in RHEL6 is older; so if we run on old sqlite, keep checking the constraints in python rather than only in SQL. Closes #10927494

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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 ')