pushtotalk: relative import boilerplate

This is necesarry to make the script executable with python pushtotalk.py
as well as python -m googlesamples.assistant.grpc.pushtotalk

Change-Id: If66aafa29ad6a03fb0fbe6e66af5a663f108fe2c

Author: proppy

google-assistant-sdk/googlesamples/assistant/grpc/pushtotalk.py

@@ -28,10 +28,14 @@
 from google.rpc import code_pb2
 from tenacity import retry, stop_after_attempt, retry_if_exception
 
-from . import (
-    assistant_helpers,
-    audio_helpers
-)
+try:
+    from . import (
+        assistant_helpers,
+        audio_helpers
+    )
+except SystemError:
+    import assistant_helpers
+    import audio_helpers
 
 
 ASSISTANT_API_ENDPOINT = 'embeddedassistant.googleapis.com'