Only run macOS backend tests when the backend is viable. Fixes #529. F…

…ixes #547.

keyring/backends/macOS/__init__.py

@@ -28,6 +28,8 @@ def priority(cls):
         """
         if platform.system() != 'Darwin':
             raise RuntimeError("macOS required")
+        if 'api' not in globals():
+            raise RuntimeError("Security API unavailable")
         return 5
 
     def set_password(self, service, username, password):

tests/backends/test_macOS.py

@@ -1,16 +1,14 @@
-import sys
-
 import pytest
 
+import keyring
 from keyring.testing.backend import BackendBasicTests
 from keyring.backends import macOS
 
 
-def is_osx_keychain_supported():
-    return sys.platform in ('mac', 'darwin')
-
-
-@pytest.mark.skipif(not is_osx_keychain_supported(), reason="Needs macOS")
+@pytest.mark.skipif(
+    not keyring.backends.Windows.WinVaultKeyring.viable,
+    reason="macOS backend not viable",
+)
 class TestOSXKeychain(BackendBasicTests):
     def init_keyring(self):
         return macOS.Keyring()