Added .viable to KeyringBackend to easily indicate if a backend is viable in the current environment

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Use .viable in Gnome and KDE tests

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Fix logic in 'viable'

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branch : prioritized backends
Incorporate DISPLAY environment variable into the viability and priority of SecretService.Keyring

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branch : prioritized backends
Fix SecretService viability tests

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branch : prioritized backends
Updated test_multi tests

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branch : prioritized backends
Fixed behavior when no exception was raised in ExceptionRaisedContext

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branch : prioritized backends
Renamed test method for consistency

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Move another delayed import to the header; fixes UnboundLocalError

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branch : prioritized backends
Use pyfs.BasicKeyring.viable to detect viability

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Fix indentation

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branch : prioritized backends