Branching for prioritized backends work

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Initial implementation of prioritized backends

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extra : rebase_source : 2b4ef6b322c081ccff6b8146c9e68bb693a0e408
More backends support priorities

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branch : prioritized backends
extra : rebase_source : 22f7e657ec4e77e82f26a3724499b6b5c7934e1b
Fixed minor issues with get_all_keyring

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Use priority for sorting keyrings

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Merge with default

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Fix reference to win32cred

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Fix improper test (used wrong name). Thanks to Dmitry Shachnev for the catch.

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Merge with default

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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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Incorporate DISPLAY environment variable into the viability and priority of SecretService.Keyring

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Fix SecretService viability tests

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Updated test_multi tests

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Fixed behavior when no exception was raised in ExceptionRaisedContext

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Renamed test method for consistency

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Move another delayed import to the header; fixes UnboundLocalError

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Use pyfs.BasicKeyring.viable to detect viability

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Fix indentation

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branch : prioritized backends