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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.