api: properly close the event loop

Close the event loop when the context is exited, which will clear
the internal queues and shut down the executor of the event loop.
Not doing this will create a warning when the object is garbage
collected.

osbuild/api.py

@@ -70,3 +70,4 @@ class API:
     def __exit__(self, *args):
         self.event_loop.call_soon_threadsafe(self.event_loop.stop)
         self.thread.join()
+        self.event_loop.close()