Keyring

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* 549: EnvironCredential now allows for equality
comparison.

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* 529: macOS backend is no longer viable if the API module
cannot be loaded. Prevents "symbol not found" errors on
macOS 11 (Big Sur) and later when a "universal2" binary
is not used (available for Python 3.8.7 and later).

* 547: Tests no longer attempt to run macOS backends even
on macOS when the backend is non-viable.

* 542: Change get_credential to return generic Credential.

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* 530: In libsecret tests, use a session collection to
allow tests to pass on Debian.

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* 521: Add libsecret backend.

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* 519: macOS backend APIs updated to newer, non-legacy
APIs.

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* 504: Better error with invalid parameter to init_keyring.
* 505: Nicer documentation for headless Docker.