libsecret: skip if there is no secret service running

In case libsecret and its Python bindings are available the
libsecret backend will get selected. This doesn't mean though
that the system service it connects to is available.

This leads to the backend being used and then failing, for example
in get_credential() with:
g-dbus-error-quark: The name org.freedesktop.secrets is unknown (2)

To avoid this ask libsecret to connect to the system service first
before considering to use it.

Fixes #600

keyring/backends/libsecret.py

@@ -7,7 +7,6 @@
 from ..errors import (
     PasswordDeleteError,
     PasswordSetError,
-    ExceptionRaisedContext,
     KeyringLocked,
 )
 
@@ -50,10 +49,15 @@ def collection(self):
 
     @properties.classproperty
     def priority(cls):
-        with ExceptionRaisedContext() as exc:
-            Secret.__name__
-        if exc:
+        if not available:
             raise RuntimeError("libsecret required")
+
+        # Make sure there is actually a secret service running
+        try:
+            Secret.Service.get_sync(Secret.ServiceFlags.OPEN_SESSION, None)
+        except GLib.Error as error:
+            raise RuntimeError("Can't open a session to the secret service") from error
+
         return 4.8
 
     def get_password(self, service, username):